There could easily be stuff on your drive that you don't need that is taking up space. Stuff like funny videos from you tube, logs, caches, demos, Garage band loops, games, music, printer drivers, extra languages--stuff like that. You may be able to reclaim some space by getting rid of some of these or moving some big files to an external hard drive.
There are a couple of utilities which can help you see what's taking up space:
WhatSize:
http://www.id-design.com/software/whatsize/
Disk Inventory X
http://www.derlien.com
Disk Inventory X is free. WhatSize used to be, but I understand there is a small charge for it now.
One caution. If you decide to remove something, be sure you know what it is. There are some programs that you don't use, but your computer does. But the big things will usually be media files of some sort like Garageband or iMovie or iPhoto. And sometimes there will appear a mysterious folder taking up space that was somehow generated. If in doubt, the genius at the Apple Store should be able to tell you what's what.
Good luck!