The International Council on Systems Engineering (INCOSE) publishes a very nice comparison of requirements management tools by feature-set, in Excel format:

http://www.incose.org/ProductsPubs/Products/INCOSERMToolSurveyConsolidatedResults.xls

I've used a number of tools and have my own preferences - I find that some tools claim to be 'requirements management' tools but are in reality just requirements repositories. 

As a consultant, my choice of tools usually comes down to the tools that are available at my client's organization.  Most of these requirements management tools are enterprise products (not just a matter of installing them onto your desktop off a CD and going from there) - it usually takes someone familiar with the tool to setup, configure, and continue to administer it on a central server. (Most have some form of browser-based access and/or desktop client access).

Outside of requirements management tools, there are business process modeling tools (Enterprise Architect, for example) - but as above, these are an enterprise / organization investment.  So the old-fashioned tools (when you can't get post-it notes and scotch tape) like Visio still work well.

There are also tools for creating nifty screen mockups like Balsamiq's Mockups, and there are some freeware tools as well.  I'm not as familiar with these (outside of the Balsamiq product), and would be interested to hear about people's experiences with these types of tools.

Sandy