So Many Option, Perl, Bugzilla, Commercial (Score 1)

There are a number of options depending on the effort you want to expend.

On the free side, definately look at Bugzilla, from Mozilla.org. In many respects defect tracking and help desk is very similiar, so a customised Bugzilla could do the trick.

You could code your own Helpdesk software, which is what Australian ISP connect.com.au have done, using perl.

If you are a glutton for punishment and seeking mega kudos, then perhaps try getting the NCR (AT&T) MP-RAS version of Remedy to work. I assume you would need to do something similar to what happened with the SCO version of Oracle on Linux before Oracle did a Linux port