First thing I do when I get a new machine is a clean install. That gets rid of all the junk and lets me decide what I put on. After installing all my apps and getting set up, then I run Monolingual for languages, and Intel code on a PPC system. After that, I get rid of Help files, PDF's I don't need, demos, tutorials, Apache, etc. I also use iphoto diet, which hasn't been updated for 6.
I'm pretty militant about keeping things trim and it saves me quite a bit in space.