Cerberus Helpdesk is one of the most popular, but can be pricey. There's also Deskpro, but it's not updated for a long time now. H2desk is also good I hear.
The only complaint is they had trouble in the past with the licensing server, but this has since been fixed. You can frequently catch their licenses on special sales.
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I second that, Kayako is a very nice system, and Version 3 has a very nice setup for multiple staff members.
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