Tender (http://tenderapp.com/ ) is pretty nice, and it looks like it's almost always cheaper than Zendesk. (I think the only time Zendesk would really be cheaper is if you're a 1- or 2-person team that can get by on the starter plan.)

As a support person, I don't find Tender especially amazing, but it gets the job done and never really gets in the way. But, more importantly, as an end user, I find Tender to be much easier and more pleasant to use than Zendesk. I cringe whenever I go to someone's support site and it's Zendesk.