IT helpdesk ticketing software suggestion

http://www.spiceworks.com/free-help-desk-software/ is a very nice program that is free as long as you don't mind advertisements.

i have used OSticket, RT i.e.resource tracker and glpi.

If you are an IT consulting company , i will recommend RT. http://bestpractical.com/rt/


P.S. I believe in opensource

+1 for Spiceworks
You should also check out Sourceforge.net for a variety of Free/Open-Source Software.  Here's a link to helpdesk software:  http://sourceforge.net/search/?type_of_search=soft&words=helpdesk.  Most of the options there rely on LAMP/WAMP, so once you get it installed and running, the sky's the limit for customization.

+1 for Spiceworks as well I am using it for my medium size company and it fits shoes well.

another one I would suggest is called TrackIt  and has a great interface which gives you alot of choices for customization.  
http://www.numarasoftware.com/Track-It/help_desk_software.aspx

If you are looking for something highly customizable with both a Windows UI and a Web UI, I suggest you look at IssueView.
http://www.issueview.com
Right now they are offering a free 5-technician license.

I've used this suite for years. It is customizable, allows for inventory management and you can check the ticket queue through any browser connected to your network. This makes working as a desktop technician great as you can check new tickets, modify tickets and close them on the fly. No need to go back to the I.T. department.
And best of all, it's free.
http://www.oneorzero.com/

we use http://bestpractical.com/rt
RT is an enterprise-grade ticketing system which enables a group of people to intelligently and efficiently manage tasks, issues, and requests submitted by a community of users.
RT is your organization’s focal point for tracking tasks, issues, knowledge, and collaboration.
It’s easy to submit, assign, prioritize, search, escalate, and report on issues.
RT keeps track of each ticket’s full history and metadata to help your organization better retain knowledge and analyze trends. RT can track multiple projects for multiple teams within a single installation.
RT tracks critical system metadata, including time spent per action, due dates, and estimated time to completion.

i recommend spiceworks its great and its free, but we use CA, its been very great so far, i am a service desk app administator and i've used HP (which is highly not recomended). otherwise if you have sharepoint use the template of IT helpdesk which can be customised to your liking.

Everyone gave great responses but this is the one we are looking into at the moment.