Seiklus

There once was a fellow by the name of Clysm. While browsing the Internet in 2003, he discovered Game Maker--the Game Maker, in particular--and decided to fool around with it. After about six months worth of work, he created Seiklus, a simple Metroidvania with an emphasis on exploration and discovery rather than simple combat. He threw in some chiptunes, called it a night, and posted it to the official Game Maker forums and asked if people liked it.

In short? Hell yes.

In somewhat longer, the game shot to popularity with both the indie and casual games crowds, and paved the way for other exploration-heavy Metroidvania games like Knytt and An Untitled Story. With its simple but colorful graphics, simple but retro soundtrack, and simple yet engrossing gameplay, it gained a well-earned place in the indie-game hall of fame.

The story (or what there is of one) is simple. A small white fellow has been separated from his girlfriend, thanks to an unfortunate meteor crash. Now, he must seek out a series of artifacts that will help him return to her. After exploring the world and finding the way up, he now must find all the colored whisps scattered around the world, find the final artifact, and uncover all the pieces of a moon-shaped amulet.

The game can be downloaded from Clysm's website for free. (Well, it is freeware, after all...)

Tropes used in Seiklus include:

Always Check Behind the Chair: Though there's usually an eye-like marker along walls with hidden goodies, the haunted crypt notably doesn't for a couple places.
Big Boo's Haunt: The orange whisp area (the "castle"). The ghosts in it are, thankfully, harmless.
Bilingual Bonus: “Seiklus” is Estonian for “adventure.”
Enter Solution Here: The two solutions to the piano puzzle.
Excuse Plot
Extended Gameplay: Even after getting back to your girlfriend, there are still seven hidden amulet pieces to collect.
Gotta Catch Em All: The colored whisps, the artifacts, and the moon amulet pieces.
Green Hill Zone: The opening area.
Invisible Block: Present in overworld area. One artifact makes them barely visible.
Lethal Lava Land: Actually averted in the more traditional sense. There is a volcano, but there's no real "lava" to speak of. You do see a field of volcanic ash, though.
Metroidvania
Mook Bouncer: Stupid cave traps. Stupid, stupid cave traps...
Moon Logic Puzzle: The piano. Remember that rainbow-skeleton in the haunted crypt? Might want to write down the sequence of the colors. Oh, and those seemingly random numbers at the top-left corner of the glitch room? They're the order of the keys you have to hit for a chest to appear.
Rewarding Vandalism: one or two jars inside the haunted crypt hold whisps.
Warp Whistle: The final artifact, necessary to access the Extended Gameplay.
Womb Level: The giant... thing living in the underground lake.