The best free issue tracker I've used is the one on Google Code. I use GitHub for hosting, but the Google Code issue tracker has plenty of options, and can fit

The best free issue tracker I've used is the one on Google Code. I use GitHub for hosting, but the Google Code issue tracker has plenty of options, and can fit just about any bill you want it to. It also has a built-in wiki! Since there's no requirement to actually host your code there, it works out pretty well.