Then SDCC is the thing you are looking for. It works but code quality is not sufficient for people who are used to comercial grade compilers. Though it is more than ok for student/hobbyist use and educational things.

If you are only looking for a free compiler in sense of free as beer and downloading, most vendors offer versions of their products with reduced features and restrictions on code output, socalled evaluation versions. At least Keil and Tasking offer such....