Here are the ones I have worked with:
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HP Service Center. Very good and complete, but maybe a bit too much for what you're looking for. This software is aimed primarily at large enterprises. It also costs a shitload of money.
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Redmine. An open source classic, coded in Ruby. It is my main recommendation for your case. It has a very strong community and provides plugins for virtally everything you can imagine.
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JIRA is free if you use it for open source or non-commercial work. It is great, highly customizable, and opens up to the whole Atlassian workflow tool that is great to use.