In a ReddIt post attempting to find a good issue tracker, RT from Best Practical came up ( http://bestpractical.com/rt/ ). A mobile interface, time tracking, and a knowledge base are the key features in RT.
The OP looked at PivotalTracker ( http://www.pivotaltracker.com ), which provides a rather good interface for developers to pick tickets to work on while on their mobile devices too. PivotalTracker claims to fit in within agile software development processes better than other issue tracking systems. There are mixed responses to PivotalTracker with one response suggesting that the simple interface was a breeze to use while another response suggests avoiding it like the plague.
Jira, FogBugz, Fossil ( http://www.fossil-scm.org/ ), RedMine, and Trac were suggested too. JetBrains offers YouTrack for users who are more tech-savvy and prefer to use their keyboards more than the mouse or other pointing device. Bugzilla is the most popular of the lot. Mantis users posted about their bug tracker of choice too.
Finally, there was a suggestion of using an MS Excel sheet distributed by email and a Google Docs spreadsheet, which is something most experienced software engineers suggest is the way it should not be done.