Hello,
Have a look at
vTiger, it's more a CRM program but comes with a full working Support portal. Gives you the advantage of having all your customer information in one place along with trouble tickets and Knowledgebase.
Another one is
eGroupware, which is a modular system, so you can in- or exclude whatever you want.
I'm sure a search in Google will come up with lots or links, or you can have a look at the
SourceForge site.
Kind regards,
Eric