Fan Control - Increases fan speed for each increase in temperature, a must for the MBP which can get really hot. It sits in your system preferences whenever you need to change settings on it.
Perian - Lets you play almost everything in Quicktime, its a codec plugin that sits in your system preferences
Adium - The best IM application out there, tonnes of addons avalible for download on the homepage, my dock icon is the Adium icon with a black macbook
you can get one with a MBP as well but theres heaps of others to choose from as well.
iStat Menus - Not really a must have but if you're like me and like to keep an eye out on your system then get this, it adds monitors into the menu, you choose what to monitor in the system preferences pane, i have up there right now, temperature, fan speed CPU usage and network.
Gimmesometune - An addon for itunes, it inly appears in the system menu bar when itunes is open, go into the preferences for it and you can tell it to automatically retrive album artwork, whenever you play a song it automatically downloads the album cover.
Main Menu - Sits in your menubar, you can configure it so that it automatically performs maintenance in two clicks using batch tasks as well as doing a alot of system related tasks like Force Emptying the Trash, rebuilding databases and so on.
Thats all I can really think of right now but will add some more later if I can think of any but those should get you started