Great idea for a thread. I'd like to add a couple I use. Maybe I'll think of more later, but these are the most useful to me.
1. ICBAR - for closing (not just minimizing) applications, task switching and general utility.
http://Icbar.ictinus.com
2. LAUNCHER - for getting more icons on the Today screen.
http://www.scottandmichelle.net/scott/cestuff.html