Non-root:TwiDroid, handcent SMS, Iheartradio, NFL Mobile (& NFL Superfan, though that isn't doing much right now...), Wifi Analyzer, Weatherbug, DroidLight (flashlight program from motorola, uses the flash LED as a flashlight) google Skymap, GPS Status, Daily Strip (all free; although some have paid versions as well, I don't need the additional functionality). Occasionally use Pandora, the Ustream Viewer, and have a lot of other apps I use sometimes.
Root: SetCPU (pretty much mandatory if you're going to overclock, but not free), ROM Manager, Wireless Tether for Android