This hard, I can't pick just one here's the lot I use on a daily basis. Newsrob, touiteur, beautiful widgets, tapatalk, folderorganizer, adw.launcher/launcherpro.
Timeriffic - sets volume levels, wifi, etc...based on timed event settings. I have it configured and totally forget about it. Phone never rings during work hours (vibrates only), Wi-fi comes on when I get to work and off when I leave, the phone goes to silent during church hours, etc..
ADVANCE TASK KILLER !!!!! MUST HAVE
Twidroid
Facebook for android
Meebo IM (gives multiple forms of IM services)
NFL Mobile
Magic marker (keeps kid happy at the doctors and such)