It absolutely depens on what your goals are. The Bugzilla and Trac systems mentioned are nice but geared towards bug tracking, which is just very different from a tool you'd want to use in a helpdesk-type setup where end users would raise incidents.
If the latter is what you are looking for I'd suggest you take a look at OTRS which is a very capable trouble ticketing system. It also has ITSM extensions, which makes it able to support ITIL processes if you need to.