I've come across two JavaScript games in the past couple of days that have really impressed me:

1. Lemmings! Seriously, it's nearly identical to the original lemmings game. It can run a bit slow but otherwise it's really impressive (via jim)
2. A 3D Dungeon game from brothercake. This is a really cool usage of CSS border polygons and JavaScript to make a 3D world.

Just a few fun examples to demonstrate the potential of JavaScript and CSS beyond Ajax, drag-and-drop and yellow-fade techniques - though of course nothing beats Buzzword Hellfire ;). Anyone have any more?