Hey there
I asked a similar question a while back on the Yahoo Requirements Engineering group.
The question and responses are listed here. There are a few around, and there are some with some cheap licences also.
As for Bas' responses, I know and also recommend reading the Jiludwig site. It's a good intro to requirements management.
And remember - It's fine to have a tool, but unless you know how to use it you end up wasting a lot of time and effort. An expert with an excel sheet is more effective than a dozen uninformed analysts with the best tools.
Lastly, we would love to get your feedback on the tool you select and use. Please share your experience with us here.
Thanks for visiting and asking.
Craig Brown,
BetterProjects.net
Modern Analyst Core Team Member