Why has no-one suggested looking at home folder and sorting folders by size? Make sure "Calculate all sizes" is selected in View Options (apple J).

That will show you where the big folders are hiding. Hey presto!

C

It will show where some big folders are, assuming the problem is there and there are a lot of file occupying drive space. I’ve been reading elsewhere that log files can balloon, and the option to make local Time Machine backups on laptops will also swallow up disk space. This being a laptop may be the cause. Use Time Machine preferences to switch this option off.

Do you have Time Machine mobile backups enabled?

I'd check /Volumes/MobileBackups/ and /.MobileBackups/.

I'd also consider searching and then asking here.

Other could be any of the following...

Google Earth cache (this can be huge. In the hundreds of GBs).
Videos or DVD rips that OS X doesn't count in the movies bit (maybe torrent stuff)
Zip files and compressed folders
Backups
Documents
Games that aren't counted as "Apps" (maybe Steam?)
Internet caches

You can get this... http://www.jgoodies.com/freeware/jdiskreport/ and it will tell you where all the size is in you directtories so you can have a look and remove it if necessary.

I use an app called 'What Size' to help root out larger files - it scans through the whole hard drive highlighting the "gert big uns" (as we say in Somerset.) You can also delete them from the app, and find duplicates I believe... surprising to find cache folders that run in to GBs... my Bridge cache folder gets pretty full.

Might be of some use? http://whatsizemac.com/

I use Whatsize as well. It is very good, though Finder can help sort out the big folders. There is also Grand Perspective which is free.