Shadowgrounds

Shadowgrounds is Top-Down Shoot'Em Up developed by Frozenbyte software (creators of Trine) and originally released in November of 2005 in Finland and Germany, making it's way to the rest of Europe and North America in the ensuing months. It became available on Steam in May of 2006.

It is the year 2096. The place is a terraformed Ganymede, site of the small but successfully colony of New Atlantis and a highly secure military facility. You are Wesley Tyler, former security officer who now works as a mechanic at the local repair station following a major accident at the plant he used to work at. He's hard at work one night fixing a truck, but after a mysterious power outage, Tyler takes his two technician friends to the nearby power facility. Two dead technicians later, Tyler's landed himself in a full-on Alien Invasion.

The player proceeds to rescue survivors, acquire successively bigger and nastier guns and attempts to find out the truth behind the alien attack.

The game could be played alone or cooperatively.

A sequel, Shadowgrounds: Survivor, was released in 2007 and tells the story of several other people trying to survive the same invasion: IGTO Space Marine Luke Giffords, Pest-control worker Bruno Lastmann, and IGTO Special Forces Sniper Isabel Larose. As the name might suggest, it added a Survival Mode to the proceedings.

Tropes used in Shadowgrounds include:

Action Girl: The Corporal you rescue early on.
A Space Marine Is You: For the most part. Played completely straight in Survivor with Luke, who is in fact a Space Marine.
Alien Invasion
Always Night: Apparently, Jupiter's light is insufficient.
Apocalypse How: Class X-2. The second occurrence was stopped at the last minute.
Antimatter: "Gifted" to us by the aliens, who did not at the time realize the danger it posed.
Bald of Awesome: Bruno.
Bullet Time: Isabel's Special Attack. It's called "Killing Spree" and keeps a running tally of how many kills you make for the twelve seconds it stays on.
Cat Scare: Happens to a scientist who thinks the aliens are breaking into his secured lab, when it's only his hamster.
Check Point: If killed, you return a safer location within the level.
Colossus Climb: Luke's special attack. Time freezes while he takes down Brutes using this method and a pistol.
Computer Virus: A virus was uploaded to the ISE Rcom systems, and gets purged by rebooting the mainframe.
Cutscene Drop: Only a few inches.
Deadly Gas: The Grenade Launcher's Secondary Fire shoots a gas grenade. Bruno can throw gas grenades.
Death of a Thousand Cuts: The bosses.
Deflector Shields: On some enemies, though they really just act like more health.
Do Not Drop Your Weapon: Played straight with humans, and justified with enemy weapons being surgically attached.
Enemy-Detecting Radar: Functions as a motion tracker. You always have it in the first game, but must buy it with skillpoints in Survivor.
Energy Weapon: The Laser Rifle, as well as the Lightning Gun.
Evolving Weapon: Collect randomly-dropped "Upgrade parts" to buy upgrades for weapons. Each weapon has unique upgrades, but one of them is always a Secondary Fire.
Everybody's Dead, Dave
Exploding Barrels: Often in densely-packed clumps for extra boom.
Fetch Quest: Rescue civilians in New Atlantis, then fetch the soldiers in the various outposts
Fire-Breathing Weapon: The Flamethrower, of course. It can be upgraded to dump unlit fuel on the ground, allowing you to light it at will and create a fire trap.
Freak Lab Accident: In the first story's background, where Tyler takes the blame since he was on the security team.
Game Level
Gameplay Ally Immortality: During Escort Missions, making them, well, not really Escort Missions. In a pinch, you can use you invincible allies as impenetrable shields.
Gameplay and Story Integration: Isabel mentions that she counts her kills to help keep calm. When playing as her, there's a counter tracking her kills. She even says "Don't interrupt me, I'm counting!" when first radioed by McTiernan.
Gang Up on the Human: Usually aliens attack you, but sometimes get distracted by turrets or allied troops. Not for long, though.
Gatling Good: The minigun is devastating, but makes you move very slowly while in use. On the other hand, it can be deployed as a Sentry Gun. It's also Bruno's best weapon, and unlike Tyler, he isn't slowed by it at all.
Genre Savvy: Tyler is a subtle example. After a few levels, he seems to take his One-Man Army status for granted, and by the endgame he fully expects to be sent in alone against overwhelming odds.
Harder Than Hard: Two additional Difficulty Levels if you complete the game. This only justs health, and doesn't affect other parameters.
Husky Russkie: Bruno, from the sound of it. Also possibly a Cool Old Guy, given that he's 49.
I Ate What?: In Survivor, Bruno takes a swig of his flamethrower's fuel. It's also implied that he starts eating the aliens when he runs out of food.
Insurmountable Waist-Height Fence: All over the place.
Interface Spoiler: The load game screen shows how many missions there are.
~It's Up To You~
Kaizo Trap: After the final boss battle, it's possible to die after the cutscene showing the enemy being defeated.
Kleptomaniac Hero
Level Editor
Loading Screen
Loads and Loads of Loading: In Survivor, inexplicably.
Lost Forever: If you miss one of the more powerful weapons (either by choice or accident), you don't get a future chance to grab it.
Mini-Mecha: Tyler fights some, Isabel gets to drive one. All of them mount twin gatling guns, Isabel's gets a Grenade Launcher for good measure. If you choose Luke or Bruno for the final levels they also get a crack at it.
Multi Mook Melee: Though not on the scale of, say, Alien Shooter. These often take the place of Boss Fights in Survivor.
Musical Spoiler: Heavy metal is always a sign that a battle is about to start. This is fine, except for one part, where what would be a harrowing battle with invisible Scytheworms is given away by the pounding music that strikes up as soon as you walk in, before.
No Kill Like Overkill: The shotgun's assault fire. Two or three shells takes care of most basic enemies. Assault fire blasts all twelve rounds in the magazine off in quick succession.
Notice This: Pickups are always framed by highly visible green brackets.
Non-Action Guy: The scientists.
Nuke'Em: Described as a "functional equivalent." The alternate fire of the Rocket Launcher spews all four rockets in one go, coupled with a uranium core. Makes a sizable boom.
One Bullet Clips: literal with the Railgun before its ammunition upgrade. Averted only for a few weapons.
Variations of Overheating can be removed for two weapons.
One World Order: The Earth government, called IGTO.
Orcus on His Throne: The hostilities were a last resort, but attempts at non-hostile communication were perceived as such. When the protagonists aren't hampered by the attacks, they resort to an extremely detailed explanation.
The Paralyzer: The Pulse Rifle's Secondary Fire acts like a Static Stun Gun. It's largely useless as it's painfully short ranged and only affects weaker enemies. Luke's pistol uses this, while his pulse rifle gains a much more useful underbarrel Grenade Launcher. Isabel can throw stun grenades, which are much more useful.
Point of No Return: Within some levels.
Poor Communication Kills: Attempts by the aliens to warn humans about the danger of Antimatter technology were interpreted as threats.
Randomly Drops: Upgrade parts used to upgrade weaponry.
RPG Elements: In Survivor. Earn Experience Points, Level Up, spend the resulting skillpoints on increased health, critical hits, etc.
Saving the World: Obviously this is what you're trying to do, but so are the aliens! See Twist Ending below.
The Scapegoat: Since Tyler was a security guard when the lab explosion happened, he was demoted to technician.
Secondary Fire: Each weapon has one, as an upgrade.
Sentry Gun: Tyler uses a few small deployable ones in the first game to protect an outpost from incoming aliens. Bruno gets to operate a huge one with a Gatling gun and Grenade Launcher.
Sole Survivor: Survivor is all about this.
Shout-Out: Many to Aliens, most obviously the assault rifle being called a "Pulse Rifle" and looking a lot like one, too. There's also a big one to Independence Day in the form of A small alien ship being researched by humans for a lengthy period of time and then being used to infiltrate a mothership.
Short-Range Shotgun: Averted, it has full range like other bullet weapons, but could still miss by being aimed short (via free camera mode)
Sniper Rifle: The Railgun functions as one, with the added benefit of penetrating multiple targets. It can be upgraded to have rebounding shots.
Standard FPS Guns: Pistol, Pulse Rifle, Shotgun, Grenade Launcher, Flamethrower, minigun, Rocket Launcher, Railgun, Laser Rifle, Lightning Gun.
Suspicious Videogame Generosity: Rampant.
Take Your Time
There Is No Kill Like Overkill: Think two shotgun blasts is fun? How about the entire magazine in rapid succession? Enter the Shotgun's Secondary Fire. Suffers from Awesome but Impractical, as most enemies won't survive half that much buckshot.
Ten-Second Flashlight: Which scares off the weak bug enemies and reveals the cloaked Scytheworms. Averted in Survivor.
Twist Ending: The aliens were trying to save us from their antimatter gift. They only resorted to deploying bio-engineered troops when we ignored their warnings. When that failed too, their only remaining option was to beg us not to activate the anti-matter bomb. It worked. Still, nice job killing almost an entire colony, aliens.
Unwinnable By Mistake: A door won't open later if you spawn at the wrong checkpoint. Word of God recommends using the console, but the worst result for purists is to restart the level.
Universal Poison: Isabel's pistol can be upgraded to inflict this, dealing damage over time.
Weapon of Choice: Survivor's characters have a much more limited arsenal then Tyler, but each set is suited to that character's appearance and battles. Luke is a marine, therefore he primarily uses the pulse rifle. Bruno's apparently just an exterminator (regardless of the size of the bugs in question) but wears fuel canisters around his belt and his nickname is "Napalm", so: Flamethrower. Isabel primarily uses the Railgun.
Standard Status Effects: The pistol's Secondary Fire is a low-damage shot that slows enemies. Isabel's pistol can be upgrade to do this with every shot, with no damage reduction.
X Meets Y: Check out the page image. It's really more like Alien Breed meets... well, it's a lot like Alien Breed.