One of our readers recently wrote in and asked us how he could slim down his install of OS X. One simple way is to get rid of all those languages that you don't use which are installed by default with OS X (if you hit 'Customize' while installing OS X you can really cut down on the bloat by only installing the languages and print drivers that you will use).
Monolingual is a freeware app that deletes all those other pesky languages from your Mac (just be careful not to delete the language you use because the only way to get them back is to reinstall OS X).
I just ran it on my Macbook and it freed up 2.4 gigs of space, though it did take about 30 minutes to run.
Update: Be careful to only remove the extraneous languages from your Mac using this program (as I did). Uncheck all the 'Architecture' options, otherwise you'll break Rosetta and blame me (and no one wants that).