Gemcraft

Gemcraft is a free-to-play Tower Defense series developed by Game in a Bottle and distributed by Armor Games, a major source of Web Games. It has a distinct Fantasy setting with a rudimentary storyline that varies between installments.

Generally, An Adventurer Is You, assuming that said adventurer is always a noble wizard out to reach some goal on the other side of the world, come hell or high water. In this case, come monsters, by the bucketload. You, the Squishy Wizard, must protect your tower (tower defense, get it?) at all costs, by placing towers next to, and traps on, the path leading to it. However, a key difference splits Gemcraft off from other tower defense games — instead of being able to upgrade your traps and towers directly, you must power them with gems. Gems are mostly obtained with a spell named Craft Gem (which is basically the source of the game's name), and have distinct colors (which affect the types of powers they possess) and levels (which affect their strength). By combining gems correctly, you can create better ones to better fight those evil monsters.

The eight gem colors (and their special abilities) are:

Red: Splash Damage in the first two games. Changed to "bloodbound" in Labyrinth: The gem gets extra power from a percentage of the monsters it kills.
Orange: Mana Leech. Each time it hits a monster, you gain mana.
Yellow: Multiple Damage. The first two games gave a flat rate to deal triple damage. In Labyrinth, the multiple damage rate can be ramped up as high as you can make it (starting with double damage).
Lime: Multi-hit. Starts with one extra hit, but can be ramped up to be able to hit more than two monsters.
Green: Poison. Ignores armor, so useful for armored waves as well as swarms.
Cyan: Shock. Can stop the monster in its tracks.
Blue: Slow.
Purple: Armor Tearing. Each hit lowers a monster's armor by a given amount.

This game series provides examples of:

Allegedly Free Game: Gemcraft: Labyrinth contains a five-dollar "premium" version which unlocks extra modes, skills, and battlefields.
All Your Powers Combined: Crafting multi-colored gems becomes less beneficial until you combine all 8 colors into one gem. At that point, it gains the power of all 8 gem types rather than the 3 dominant ones, and its damage, range, and firing speed all skyrocket. Averted in Labyrinth: Supergemming in this manner doesn't exist.
Damage Reduction: Armour level.
Dangerous Forbidden Technique: Labyrinth has a meta example: The glitch that allows the creation of near infinite strength gems has a chance of deleting your save file.
Chain Lightning: Lime gems can strike chain hits.
Disc One Nuke: One of the early stages in Gemcraft: Labryinth has a tower surrounded by eight amplifiers. Place a gem in each of the amplifiers, and the tower gem will kick ass. This nuke becomes MUCH more powerful if the player buys the "premium" version and uses endurance mode with this strategy to get a ridiculous multiplier to their XP gained.
- Red gems in Labyrinth. They add a percentage of their kills to their damage. Meaning a red gem that's been there since the first wave can have a damage of over 1000 by the final waves and in fact out-damage gems of other colors several grades higher than it.
Mana: This is needed to build things, craft gems, cast spells, and performs double duty as hit points as well. It builds up slowly every second, but largely comes from monsters you kill, and gets increased by using orange gems in the right places.
Level Grinding: And lots of it. In Chapter Zero, you'll have to level up to 200 in order to max out all your stats, and in Labyrinth, there is no level cap for 3 of the skills. For the premium version, the Ritual skill will let you gain one starting and maximum mana for a set number of battle amulets you possess, based on its skill level.
Multi Mook Melee: Because it's tower defense.
Nice Job Breaking It Hero: In Chapter 0, you spend the game searching for the Gem of Eternity. When you find it and capture it, it turns out to have been containing a monster (for eternity, hence the name) which then possesses you, setting the stage for the first game.
Player and Protagonist Integration: Type 2.
Skill Point Reset: You can do this for free at the skill screen.
Splash Damage: Red gems give splash damage in first two games. Lime gems give a multi-hit version (which is likely why red gems were changed.)
Sound of No Damage: The game plays a metallic clink if an attack on an enemy is nullified by Damage Reduction.