Yahoo Pipes

This completely rocks - yahoo pipes allows you to remix RSS feeds visually - in a DHTML/Flash IDE in the browser. The metaphor is Unix pipes. Setup an RSS input (URL) and then create a sequence of pipes (filters, transformers). You can then publish the result. I was able to create a pipe that filtered RSS feeds from developer.capeclear.com, to filter only XSLT related forums in about 1 minute.

Plus, it has an interactive-ish debugger. Sweeet. Check it out.