I am evaluating several bug tracking/project management software packages and have found a couple of interesting one that seem very robust and are quite popular.

Even better, I have found that two of these offer some sort of free version of their paid product, being the amount of users, the only limitation...

• OnTime 2008 by Axosoft.
  Offers a single developer edition for free (details here and here).
  
  
• FogBugz by FogCreek software.
  Offers a trial that (via workhappy.net) can be enabled to work fully functional for two users in student and startup edition  (details)