Another great discovery: Feedster lets you create a feed based on a search that is limited to the feeds listed within a given OPML file. An OPML file is better known as a blogroll — basically list of RSS subscriptions (such as the one created by Bloglines when you use it to keep track of your blog and news feeds).

So if you have a blogroll, but only want to read what those bloggers are saying about particular subjects, you can set up a Feedster search to track just their ramblings on that topic. If you’re a BlogLines user, the URL for the OPML file for your publicsubscriptions will be