For the uninitiated, Yahoo Pipes lets you take one or more RSS feeds, do stuff to 'em, and return a new RSS feed. I found that it's a terrific tool if you have some RSS feeds that you *almost* like. You can tweak with 'em a bit and return a much better feed.
For example, I like The Macrodobe Weblog Aggregator, which is to say that I almost like it. Only problems that it has is that it does a lot of duplicate posts (presumably from when people edit entries after-the-fact), and it aggregates posts about stuff I don't really care too much about, like ColdFusion. Well, after about 5 minutes with Yahoo pipes, specifically with thier "filter" node and their "remove duplicates" node, I have a much-improved RSS feed.
And if you make a feed that might be useful to others, you can publish it to the world. For example, I like AMERICAblog, as it's my favorite source for leftwing news, but the author does have a bit of a penchant for open threads and pictures of orchids (his hobby). If you search for "Americablog" on the pipes page, you can now find an available "Americablog minus open threads and orchids" feed. That's mine :)
Now then, pipes are good for much more than just filtering out stuff you don't wanna read or lumping together similar feeds. You can aggregate together several feeds and do wild stuff to 'em. A quick browse of the published pipes will show off several interesting feeds more experienced users have done, like "Penny Arcade minus news items plus images included directly in the feed", but I'm thus-far happy just being able to easily filter down my posts to a few hundred a day.
Also their debugger is cool. You can watch the RSS output at any step of the process in a little window. It's really a well-made but undeservedly obscure tool.