Another vote for RT - I've commissioned it in the past, and we were up and running pretty quickly. It's certainly comparable to the commercial software (Hornbill Supportworks) that we use at the moment.
We use SmarterTrack since its previous version (SmarterTicket). The free version supports one agent and offers all functionality of the enterprise version.
Best helpdesk / issue tracking system I've used was written in-house (ping Rob Butterworth on the members list) - it was on a LAMP stack but could probably just as easily run on WAMP. There was some discussion about open-sourcing it - not sure where that got to.
Features -
Email call logging and automatic receipt notification to the client + tracking url
Email updates automatically tracked in the web interface
Tech & Public fields
Multiple Techs & Group assignment (updates goto whoever is in the assigned group)
Sub-calls (ie for starters & leavers - new starter call from HR spawns jobs for IT, Business Services etc)
Browser based interface - works with multiple tabs
Easily create multiple instances (ie Business Services or Comms can use it to track work requests as well as IT)
Basic reporting
Clients can see their own calls and call history via browser
Sadly the OSS release of CT never happened, changes in management, etc. The system is still in use, but not by me as I moved on. The system had lots of areas that would benefit from improvements, but the day to day use was, IMO, very slick and low overhead, as we wrote it ourselves for our own use.
Since moving on, I've aquired an already established AdventNet ServiceDesk+ installation, which runs on Windows (or Linux) and to be fair isn't too bad. It actually has a lot in common with CT, and several refinements that would have made it into CT had we every carried it forward as an independent project. It's just not as slick with it's email integration (though it *is* kinda ok) and I can't get through half as many call updates as I could in CT. At first I thought this was just because I was less familiar with SD+, but after almost a year I know it's because SD+, and many other systems like it, are not carefully optimized for use in high throughput, low overhead helpdesk enviroment. Just IMO, of course ;-) Ho hum.
SD+ does have the advantage that it does hardware and software tracking too, though it's not the slickest of products for that either. It's good enough, though, and not too costly.
I've tried RT, and liked it, though it was a little picky to get working, and whilst it can be made to run in a windows environment, it's spiritual home is really *nix, and that's where it will work best.
I did a comparison a few years ago of quite a number of open source packages and ended up settling on a commercial package: HelpSpot from UserScape software (www.userscape.com).
You can get a free trial version to test on your own servers. Well worth a look.
Spiceworks is more than just Helpdesk software. It has the capability of network scans, troubleshooting, reporting, vendor purchases.Customizable to your needs. I have been using it for several months now on a Server 2003 platform, It has been a very useful tool.
I haven't really used the helpdesk facility, rather the software auditing capabilities, but we use GLPI which seems to have a pretty comprhensive system for helpdesk.