Really it depends what you want out of your helpdesk!
OTRS is a freebie and is ITIL-compliant... however it's pretty comprehensive, shall we say!
If you literally just want ticket in, ticket out, I used Trellis Desk, which is really easy to use, and with a bit of tweaking can be linked with AD :)
If you don't need bells and whistles, we're getting loads of use out of freshdesk free version Freshdesk Plans & Pricing | Freshdesk help desk software trial
We have been using this for 3 years now... WebHelpDesk Pricing is based on quantity of Technicians - no limit on clients/end-users.
They offer a server version and hosted version(its what we use, one less server to manage to be honest)
Very complete program overall. It also has asset management included.
We love our Kayako Helpdesk and have done for the last 7 or 8 years.
We use otrs -- you better know some linux though
I have been persuaded to give Spiceworks a go. If it doesn't work out [for whatever reason] we can look at it again.