Reading Alexander Limi's blog two days ago, I found an interesting article about Trimmit. It is a tool that cleans your MacOSx binaries, removing some innecesary stuff to make them smaller than the original size.
Alexander just tried the app himself, with some interesting results, like the Addressbook app ending with a size of 4Mb instead of the 55Mb it is sized by default or Mail.app being resized from 289Mb to 21Mb. The drawbacks are that not all applications could be trimmed, but most of that untrimmable apps are third-party ones (which in my case didn't waste so much space). There's a list where you can check if a given app could be trimmed or not.
That's a good way of get some free space in laptops with smaller hard drives (like my macbook with the 80Gb hard drive).