This is a topic that is been overkilled in the past as you will get a gambit of opinions - but that appears to be what you are looking for.
Personally - I like Kayako and have had pretty good experience with their support. It is rich in features and really helps with the mundane processes of tracking tickets. For some - the features might be overkill and you might not need all of them but I think for the money - its the best.
Others - Cerberus and Support Trio - I have used all and all are pretty good.
I think support software mainly depends on personal taste. The only free one I tried was PerlDesk v1 which was pretty good, I upgraded to V2 when it was first starting out and it wasn't all that good so I switched to Kayako, and now I work on Cerebrus
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