We use Bugzilla, and find it meets our needs quite nicely. It's pretty simple to install and set up - if I remember correctly, you'll need MySQL (freely available), perl, a bunch of modules (I think there's a bundle), and a webserver (apache does nicely). Oh, and a real OS (Linux, Solaris) - if you want to set it up on Windows, you're probably in for trouble. The installation section of the docs might be a good place to find out what it takes to install. You can probably get it up and running in a couple of hours. Good luck!
