Hi Jageaux
The things you might consider moving to an EHD from where you can run them when the EHD is mounted include: iTunes Library (open iTunes>preferences>advanced and it will show the iTunes Media Folder. Use Finder to locate this and copy it to the EHD). Use iTunes app to search for and link up with this copied library. Having confirmed that it works, delete the library from your Mac Int HD. This will free up a lot of space if your music library is large.
You can also move your iPhoto library and, if you have one, your Aperture library to an EHD. They will run from the EHD when linked. This should enough; but if desperate, you can move your Documents to an EHD as well. BUT leave your Home Folder where it is. Don't move anything else.
Of course as Nick and Harry say, you could pay the money and get a larger Mac HD. Whatever you do, backup using Time Machine, a cloning software like CCC or SuperDuper.
Ian