Greasemonkey is a Firefox extension that allows changing the behavior of websites through the addition of "user scripts." User scripts are also supported in the latest version of Opera, and in Internet Explorer through Trixie and the not-yet-released Turnabout. The user scripts I have installed include Jesse Ruderman's AutoLink (which creates hyperlinks in useful places), Julien Couvreur's BoingBoing Butler (which removes the advertisements from BoingBoing), and IMDBAge, which displays the age of actors on IMDB.
The RSS feeds for the Washington Post have an annoying "feature:" the links to article include a query string ("?nav="). Instead of AR2005060401226.html, the feed for the Front Page would link to AR2005060401226.html?nav=rss_print/asection. Both hyperlinks go to the same article page, but look like different pages in Firefox's cache. Since each feed has a different query string, the problem is multiplied if you subscribe to different feeds.
Updated the script to include the Opinion section.