Came across this today on GitHub (web dev here) - it's a pure JS tetris, pretty cool!

https://github.com/nicktouchette/tetris

It's a good game for how short the source code is. Rotating doesn't break the autorepeat chain and the speed level becomes challenging. However because of the way the gravity timer was chosen (500-level*30), the speed difference between level 14 and 15 is huge and level 16 is already killscreen. Also only one rotation direction and graphics are messed up in Chrome.

Other JavaScript games: Tetr.js , JSTris . And a one with retro looks.