Well, these two may not be cheap, however they are web based and I have experience with them both. Kayako and Cerebrus. Kayako is a nice system but their support sucks and Cerebrus is ok too and their support is pretty quick.
Ping Zine just ran an article on both of them and Cerbrus won it but with Kayako a close 2nd (I think
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Just FYI. Remember, you get what you pay for. Cheap does not always mean the best and if you are in somehting for the long haul you may want to invest a little more. But if short term is your goal then cheap is good too. Take care!