HI there, I know this thread is wicked old but, since I am doing this in a class I thought I would answer. You can use SPIM. It is at: http://www.cs.wisc.edu/~larus/spim.html.
Good tool, but not user friendly. But hey, assembler is not the friendliest language either. If you know the MIPS instruction set, you can deal with it, I'd gather.
To the poster above, I think he was looking for a simulated environment in which to envoke the code, for testing purposes. MIPS is in its bulk for embeded systems, which I some-what doubt have very functional compilers (there are always exceptions, I understand, but most people want a similated environment to test the code instead of fishing out MIPS PCs to develop on).