Yahoo Pipes allows you to manipulate rss feeds. I built my first very simple pipe this morning. It takes the Amazon Gold Box feed and strips out the Baby Deals. You can also much fancier stuff like match geocoded addresses in feed items and modify the content of individual items. Too bad they don't provide a way to screen-scrape pages to build feeds.
If I build any more pipes, you'll be able to find them on my pipes page.
The state is that great fiction by which everyone tries to live at the expense of everyone else. - Frederic Bastiat