This is my list of my top 3 list of available help desk software. I have created this list from my own experience working in a global organization and you should only take it as my point of view. ITIL version 3 compliance, performance, scalability and customization options are what I have been looking for in our Service Management software.
Here is a Help Desk Resume for my top 3 choices:
HP Service Manager 7.1 - a solid service management tool that offers an almost unlimited degree of customization options. Reporting, availability and scalability is maybe the best on the market. Downside of this help desk software is its complexity and price.
CA Unicenter Service Desk - software for Configuration Management and automation can be a big challenge but CA is leading the battle. Unicenter Service Desk is perhaps the most flexible tool regarding integration of the 3 mentioned here. Downside is again, complexity and price.
IBM Service Request Manager - a great service desk software, that really excels in ITIL Change Management, Release Management and trouble ticket software. Downsides are price and price of customization due to the fact that this often requires IBM consultants. If you can do with what is out-of-the-box, this might be your best bet.