Alt+Tab replacement for Windows 7?
I've used Switcher before, and it works pretty well. Requires Aero and is more of a replacement for Flip3D than the traditional Alt+Tab. Does keyboard context searching, too.
Another candidate is VistaSwitcher, from the same person that created TaskSwitchXP.
Internally, the trick seems to be that you have to use a low-level keyboard hook to listen for Alt+Tab, instead of registering it as a hotkey (since you won't be able to do so on Win7).