Lansweeper Help Desk is an excellent choice for ticketing. It is not expensive, and it's pretty easy to use. I like the custom fields, and the configuration is pretty easy.
Here are some improvements we'd like to see in future versions of the help desk:
1. The ability to turn off all outgoing email to users. We want the agents to receive ticket assignment notifications, and that's it. We don't want users to receive public notes, or when an agent responds to email they received.
2. The ability to change the source once the ticket is entered would be good. I don't see that anywhere.
3. Some sort of notification (pop-up maybe) when an agent is entering a ticket for a user, and there is another ticket open for that user.
4. Allow subsequent emails (within a certain time frame) with the same subject (maybe with the addition of Fw, Fwd, or Re) to be appended to the original ticket instead of opening a new one. This was something that Spiceworks had if you can take a look at it.