fMMS as you can't send MMS straight out of the box
rootsh for root access
Tweakr so you can edit various options and save more profiles than the standard 2
Petrovich for easy media sharing to fb, flickr etc
FMboost to boost power output of FMtransmitter
gPodder and Pannuci for downloading and managing podcasts
Mediabox as it's better than the built in player
Conky so you can look at rootfs (and aviod filling it!) and other system info
Autodisconnect to manage connection disconnects and save battery
BatteryGraph (or Battery Eye) to help see how efficent your battery is...

That's my list at the moment - I'm sure others will offer different opionions....