Edit: Oh, found it here. http://www.zendesk.com/product/compare
We use OTRS (OTRS Help Desk software - OTRS IT Service Management software - Free Open Source Help Desk - Problem Management System - Customer Interaction Software | OTRS). Very powerful and configurable, can accept email or web tickets. If you go for OTRS ITSM then you get an ITIL compliant system.
I use webhelpdesk its not free but it is what our ITC uses so that's what we got too. It used to be made my mac design studio but they got acquired by solar winds this past summer.
It is very customizable, accepts tickets through email or web interface. You can set up a number of locations and categories etc.
We use Redmine
Having looked at many of helpdesk systems (including all of those mentioned in this thread) we ended with something which isn't even really designed as a helpdesk system (it's a project management application which has issue tracking incorporated within it). We find that it is helpful to have all of our projects and support work in one system and this is something which, in my view at least, Spiceworks doesn't do particularly well.
In terms of using it as a helpdesk it has everything we needed:
- LDAP integration (if you want it)
- Web portal and email ticketing support
I did stumble across a VM appliance for RT
havent fired it up yet but that would appear to take some of the setup hassle away
might be worth a look RT 3.8.7 VMWare appliance with RTFM | Groundtactics.com
I used WebHelpdesk (now part of SolarWinds so not sure what pricing is like)
Worked a treat and very easy to use (both for Admins[Techs] and End-users