Homestar Runner: Classic flash website. (Req. Flash)
Peasant Quest: This is a great game that mimics old MS-DOS games. (Req. Flash)
AIDungeon 2: Wonderful text adventure where you can do anything you want.
Club Penguin Rewritten: quack..? (Req. Flash)
Candy Box 2: ASCII-based idle RPG adventure game!
Agar.io: Oops! All circles!
Slither.io: Oops! All snakes!
Wikiverse: Wikipedia, as a universe.
Uncyclopedia: Wikipedia, but dumber.
Nested: A (text-only) universe for you to explore.
Mazedude's Magical Website: My friends (horrible) site.
Jackomix: A generic personal website.
elementz: A modern-looking website that has cool things.
Chromata: A digital art tool that tries to replicate your pictures.