Yahoo just launched Pipes. It allows you, through a drag-and-drop interface, to create your own feeds from most any source, filter and massage the data in a number of ways, then output to rss, atom, or whatever.
You can publish pipes so they are shareable. Then others can take your pipe, save a copy, and modify it or use it as they want. While this is still fairly geeky stuff, it takes much of the grunt work out of the process.
Here’s a little Pipe I created to aggregate news on global warming, note that it displays the pipe itself too, so you can see how it works.
Yahoo has created a seriously useful app here.