i've had my 3G jailbroken for a few weeks now (just upgraded to 2.2) and it is really interesting.
I am not much of a computer nerd or geek or anything, but I find it very interesting to SSH. You can do a LOT with SSH, get emulators, customize themes (more then you can do with just a regular theme - see below, i combined 2 themes). One of the coolest things is changing the actual background without having to change the lockscreen image (it takes a little more time, but worth it i think) 😎
video recorder,
MMS,
ROM's,
winterboard,
cracked apps (the ones you find in the AppStore)