Knytt Stories

A freeware platformer game by Nicklas 'Nifflas' Nygren, creator of similar freeware platformers such as Within a Deep Forest and the original Knytt. Though it only comes with one full level and a tutorial, the game has an integrated level editor and new levels are very easy to install. Several additional official levels have been released (see below), and the game has a very active fan-made level community. Dozens can be found on Nifflas' forums and on knyttlevels.com.

Knytt Stories has been billed as an "environmental platformer". Though enemies and obstacles exist which can kill your character, a tiny humanoid named Juni (whose backstory tends to change even between the official levels), the ambient/playground levels tend to be the most popular, thanks to the gorgeous music and sprite packs - both the originals and those added by the level community.

Official levels (made by Nifflas) include:
 * The Machine: a puzzle platformer based around finding upgrades and exploring the world. Has a bonus ending if you find all four keys.
 * A Strange Dream: a huge, gorgeous Wide Open Sandbox that can be finished in about half a dozen different ways. Contains some bonus material if you know where to look, and a secret Mythology Gag ending.
 * An Underwater Adventure: survival horror platforming (no, really). Has two parts - once you finish the first run, go through it again for the second half. Also features multiple endings and some very nice puzzles.
 * Sky Flower: climb upwards, float downwards.
 * Gustav's Daughter: a relatively short level with more experimental graphics, mostly built around jumping/climbing skills.
 * This Level Is Unfinished: Exactly What It Says on the Tin, hilariously so.

Available here.

This game has examples of:

 * Continuity Snarl: Justified given that most of the content of the game is fan-created.
 * Double Jump
 * Heroic Mime: Juni.
 * Averted in several fan levels.
 * Hologram/Invisibility: The hologram powerup makes Juni invisible and plants a decoy hologram that attracts enemy bullets.
 * Invisible Block: Some blocks aren't visible until you get close to them. Some blocks are visible but aren't actually platforms. Some blocks don't even exist unless you have a certain power-up. And some blocks aren't visible at all.
 * Multiple Choice Past: Juni.
 * One-Hit-Point Wonder: Juni.
 * Parasol Parachute: Juni's most characteristic powerup.
 * Save Point
 * Spikes of Doom
 * Super Drowning Skills: If Juni touches any liquid, she dies.
 * Justified when the water is colored to look like lava or acid.
 * Wall Jump

In addition to the game itself, some levels have examples of:

 * Arc Words: This is what happened next:
 * Back Tracking: Sometimes essential to complete the level.
 * Bonus Dungeon
 * Bubbly Clouds
 * Death Is a Slap on The Wrist
 * Ghibli Hills
 * Metroidvania: most levels are like this.
 * Multiple Endings
 * Mushroom Samba: The level aptly titled "Don't Eat the Mushroom". If you do, you go through a series of trippy scenes from
 * Mythology Gag: The alternate endings of some of the Nifflas-created levels. One of them puts Juni at the beginning of Within a Deep Forest; another makes her reenact the opening cutscene of Knytt. There's even one level (can't remember if it was third-party or not) that puts you at the entrance of #modarchive, in a reference to one of Nifflas' early games, aptly titled #modarchive Story.
 * Nintendo Hard: Some levels have multiple difficulty settings.
 * One word: Xeia.
 * Once an Episode: You will find an obstacle which can only be passed by running, one which can only be passed by double jumping, one which can only be passed with the umbrella...
 * Save the World
 * Wide Open Sandbox and possibly Quicksand Box