I mainly use Mail, iCal, Address Book, Safari, so that's easy as they come with it.
The essential non-standard apps for me are MSN Messenger, NewsTicker (RSS client), iView Media (photo organiser) and PhotoShop. I also use MS Office but could probably live without it.
I use Retrospect for backup, and handy utils are Airfoil (AirTunes from any app), BluePhone Elite (text messaging) and Chicken of the VNC (VNC client). Toast is handy on occasion, so is RealPlayer.