Assuming you use Windows:
Check any folder like C:\TEMP C:\WINDOWS\TEMP and delete everything deletable. Sometimes download programs or installs accidentally leave copies of things in those type of folders. Also clear your browser cache's. Plenty of instructions in browser's help or all over the web on that. Even if you don't use IE, open it and clear it anyway, there could be old unneeded files in there.
You can compress any folder that you do not use often, including folders for applications you rarely use. This will slow down your use of the folders you compress, but since you rarely use them it should not effect you. Assuming you have Windows, you can right click the folder, choose properties, and click the Advanced button and choose "compress to save space". Your mileage may vary, but this will allow you to keep the files usable but save a little space.
Of course, there are cheap external HD's out there, as well as memory sticks, etc. for temporary storage of things you can move off the laptop. I keep all my media on an external 120GB, and then only copy shorter random playlists and a couple of video files I'm watching, etc to my portable devices.
