Opera built-in RSS reader is the best I have seen. It display rich contents properly, does not slow down if you have thousands of entries in the feeds (unlike many of the others listed above) and has a good user interface.

I've tried Sharp Reader, RSSReader, the Sage extension (and a few more other extensions), RSSBandit and FeedReader. None of them is as good as Opera RSS reader. Most of them chokes the system if you doesn't prune your feeds (which I usually didn't do). Only Opera can handle feeds with thousands of unpruned feeds.