Depends on how much you want to do. The Keil compiler is OK, but you will need to pay good money for an unrestricted (floating point etc. included) version.

If you're only writing a couple of k of code, and don't need floating point etc, you can get a good assembler for nothing. I find this is fine for what I do at home, but at work where things are more complicated, I use Borland to program in C.