If you are using a Mac with a small-ish SSD these days, there's a good chance that you are running low on disk space. A fair bit of an OS X install is multilingual files and if you are using just 1 or 2 languages, you could get rid of the language support files for all other languages.
Enter Monolingual, a tool which cleans up all the languages that aren't of interest. On an OS X Lion install, that's worth at least a couple of GB.
Monolingual can also remove files for architectures that aren't in use (e.g. getting rid of ARM and PowerPC files on an Intel Mac.
Download Monolingual here.