Gimp is a nice free, if less powerful, substitute for Photoshop.
Rightzoom is neat tool that makes the Zoom window button work like the Windows Maximize button.
Tinkertool lets you customize several system settings.
Quicksilver is an AMAZING launcher and more.
Flip4Mac, Perian, and VLC will let you play just about any kind of movie file not supported by Quicktime.
Disk Inventory X is a neat way of looking at how your hard drive space is being used.
iFreeMem is a handy tool that lets you free up RAM on the fly.
GetLyrical is a great way to get lyrics to most of the songs in your iTunes library.
iShowU is a pretty complete screen recording application.
AppFresh is a good way to make sure all your applications are up to date.
LiteIcon lets you easily change your system icons.
Tooble lets you download YouTube videos.
Journler is a great journal-writing application.
WebRemote lets you control YouTube videos with your Apple Remote.
iAlertU is a neat security alarm for your Macbook.
Orbicule Undercover is supposed to be a great way to track your MBP if it's stolen (luckily I haven't had to find out).
MindNode is a good way to map out ideas.
aTypeTrainer4Mac is a cool typing app if you need to brush up on your wpm or want to teach someone else to type.
If you write scripts or screenplays, CeltX contains templates for them.
Frame by Frame is a free stop-motion animation program.
That's all I can think of for now.