Torus: Another Tetris-alike, the difference here being that instead of being a bordered plane, you can cross from one side to the other, and lap pieces over the border. Oh, and there's no border, because it's actually presented in 3d!

It's really a simple version, but, niftily, it is not implemented in Flash or Java, but all in totally standards-based HTML5. Which means that you'll need a modern browser to run it: pretty much any current-release browser except IE8. (It might run there, but I doubt it, and I haven't checked.)