We're not quite ready for prime-time, but my startup (Mad Wombat) is developing a helpdesk application called Tracker, which might suit your needs, provided you don't mind us still being early in our beta.

Tracker is a very tightly focused piece of software; it doesn't have public forums, or a wiki, or a knowledgebase. It just tracks problems, which can be submitted via email, via the web interface, or via the REST API.

Now keep in mind, it's still early in our beta. The major features all work, but there are plenty of bugs, which we're fixing as fast as physics allows. Although we do use Tracker as our own customer support and bug-tracking application, so we've got a lot of motivation to make sure that it works.

Shoot me an email if you're interested: don@madwombat.com