Okay, just because everyone else is being vague and saying "everything" or recommending other computers, I'll actually say what I do with my MacBook 13".

I use my Macbook as my main computer, because I don't have a desktop that was made after 1999. It works very well, and I bought a small STM shoulder bag for it so I can take it places. I don't bring it EVERYWHERE with me (usually not to school) but when I'm at home I use it for hours. Some specific things I do:

1. Media - holds all my music/tv shows/movies
2. School - iWork for presentations and essays etc.
3. Games - I mostly play Call of Duty 2 online, which runs flawlessly.
4. Organization - iCal + Address Book = very handy way to stay organized. I also use an app called Aurora (freeware) for my iTunes-integrated alarm clock.
5. Communication - iChat, Skype, Mail.app, Firefox.
6. Web development - CyberDuck/Transmit for FTP, GarageBand for podcasting

And in general, though it does sound cliché, I really do use my Macbook for EVERYTHING. I'm on it probably an average of seven hours per day. There are just so many applications I use to make things generally easier.

By the way, here's my general Macbook/OSX setup: Click

Any questions, shoot me an email or PM.