Like others here I use either a text file or the bug tracker built into whatever dvcs hosting service.

Lots of it depends on what sort of "personal project" it is. Is it something that is ever going to see the light of day or is it just an experiment? Is this project in use by the public?

For example, one of my personal projects became moderately popular and setting up a Get Satisfaction site for it worked really well. Not really "bug tracking" but it worked great for bug/feature requests.