ultrasn0w for 3G, Winterboard, SBsettings, NES, OSU, SSH, MXTube, etc etc. I would say ultrasn0w, SSH and Winterboard are the 3 main important reasons why people jailbreak.
ultrasn0w is a software unlock for the iPhone 3G
Winterboard allows themes.
SSH allows you to wirelessly get into your iPhone.