There is a video of Compiz Group Tab here: http://www.youtube.com/watch?v=1nK4_cH5sbM&feature=relat...
Focus on mouse comes in several flavours, the best seems to be sloppy focus with raise-on-focus disabled. When you mouse over a window it goes active so you can type in it. Because it doesn't come to the front unless you click on it you can leave your IM window/IDE/whatever half under a browser window and still type into it. Truly a killer feature, especially on laptops where space is limited.