Super Mario Bros. Crossover 2.0 is out! An expansion on the original game, which let you play as various NES characters transplanted into Super Mario Bros., but using the rules and abilities of those characters from their original games, version 2 offers more special abilities, more characters, and your choice of audiovisual "skins" based on four Mario games from the NES, SNES, and Gameboy, along with one based on Demon Returns. There's even instructions for playing with a gamepad! For more information, see the Super Mario Bros. Crossover Wiki or watch the exciting Super Mario Bros. Crossover trailer! [Previously]
Super Mario Crossover - Play Super Mario Bros. as Bill S, Samus, Link, Simon, Mega Man or, of course, as Mario. [more inside]
Do Do Do Do Do Do Do Do Do Do the the the the the the the the the the Mario! Mario! Mario! Mario! Mario! Mario! Mario! Mario! Mario! Mario!
Enough Plumbers (Flash game), a nice twist on a classic game: "It's when the plumber encounters the coins, though, that everything changes. For every coin you collect you also get a cloned plumber who copies every move of the original. Just trying to jump ten hapless clones at once onto a small platform and you can see how things have changed." (review) (via waxy.org)
Metroid Cubed...Squared! It was linked a couple years ago, but since then the site has undergone a major overhaul and added a bunch of cool stuff. [Flash Warning] In addition to the now classic voxel 2.5D conversion of Metroid (playable and interesting), there is now a robust Metroid world-editor, a Marble Madness-like prototype using the voxel engine, a StarFox level editor and model viewer (Spoiler: gfx not as good as you remember), and numerous others worth checking out: Hyrule Planetoids (comme Le Petit Prince) and a OpenGL Zelda room viewer, a Mario 64 Explorer, and numerous Super Mario Bros demos. It's all quite useless/amazing.
Super Mario Brothers sound effects. Boing! Boing! CRUNCH! Ding! Ding! Ding! Whoop! Boing!