For managing requirements in an agile environment, Plutora's RTM is a pretty powerful solution. Customizable, interactive, real-time views of the testing process, plus it's easy to pick up and use. There's a whole slew of these types of tools, but depending on the scale of the project, it might make sense to stick with tables and spreadsheets.

I don't think agile is inherently incompatible with RTM, as a previous poster suggested.