My vote currently is for Eventum. Whilst I wish it were in Perl, it seems to be pretty good quality PHP. There is an active mailing list and ongoing support and development from within MySQL. I use it on Linux (CentOS 4.3), it has a very quick and easy automated installer process and was very fast for people to learn. We have installed it for one client who use it as a 'Feature request' and 'Job Management' system as well as for issue tracking. In six months they have logged about 18,000 issues with bascially no problems.

I also looked at Trac (as suggested by radiantmatrix) and whilst the idea of integrating source control is nice, there use of SVN made it troublesome as we are a CVS user (one of my contractors is a CVS maintainer so I am kinda stuck!).

Bugzilla is worth a look, it is big, and as Perrin so subtely reminds us, is not simple. But it does work well in places where I have seen it. There are some bugzilla users here in the monastery.

jdtoronto