While you're at it try SDCC. http://sdcc.sourceforge.net

It's opensourced and gets the job done. If you're looking for support you're on your own (RTFM like they call it in opensource land).

We use Keil at the University but because I don't have that much money I use SDCC at home. I haven't noticed any big difference between the outcome of the two. Ofcourse SDCC doesn't have a fancy graphical interface and doesn't have a huge debugger like Keil.