A great comparative resource for prototyping tools. Very nicely presented by usertesting.com
If you're dipping your toe in the water with prototyping and moving beyond pencil and paper, this is worth a quick look.
I've only tried a fraction of the tools listed here, encountering their pros and cons. I know what I like now and what I don't, but it's good to get an overview of what is an ever-changing market.
(University of Edinburgh staff remember - we have Balsamiq integrated into the wiki service and free to all!)
Usertesting.com's The Ultimate Wireframing Tools Guide