1. Quicksilver - I use it so much I almost think of it as a part of OS X whenever I work on anyone else's Mac I am always hitting control + space trying to bring it up and realizing it's not there
2. Skitch - I have been using it since t was only a private, invitation only beta. I use it all the time and love it! Simplifies screen shots so much and provides a simple photo upload option.
3. Photoshop CS3 - I do image editing, it is amazing
4. Office 2008 - I'm a college student, couldn't live without it, the notebook view with audio notes is a life saver
5. Adium - A great chat application
6. Firefox - The only browser I really like.
7. InsomniaX - While it is not fuly functional on Leopard it still does what i need, and is very useful to me.
8. GrowlTunes - Amazing plugin for iTunes
9. iMovie '06 - I love editing video, and this is a simple solution
10. Missing Sync for Windows Mobile - Easily syncs my phone couldn't live without it
11. LogMeIn - Great remote desktop app, also allows me to control my laptop from my phone, I often connect to my computer through LogMeIn on my phone and use my phone's keyboard to type.
12. Parallels - I am an Information Assurance student, so I need to do a lot of hacking, most apps and methods for hacking require windows and various forms of Linux which I can run virtually (integrates great with spaces 4 different OSs running at once all on different spaces
the power of a MBP gotta love it)
13. Nocturne - This is great when you work with your laptop in the dark. I use it very often in class to take notes during a lecture when the lights are off, a laptop screen can often be blinding in a dark room. Nocturne uses the light sensors to make the screen monochrome and gives it a red tint making it much dimmer yet still very visible. It is also very customizable. It is made by the same people that make Quicksilver.
These are just my favorite apps, I could go on forever.
