Russell Dickenson wrote:
[...] Mantis - http://www.mantisbt.org/ - looks good and is seemingly actively developed. [...]
I don't know how "active" that really is, but it's good old basic PHP on top of MySQL and thus will run pretty much anywhere -- I have it set up on some really cheap hosting for some projects and it basically just works. It doesn't let you merge duplicates, but it's very easy to mark a bug as a duplicate (as well as add relationships to other bugs, and links are as simple as writing #123 in a field to link to bug number 123).
It's ugly, and probably not as flexible as some, but it doesn't ask for much (i.e. hosting support) and is easy to set up.