RSS Micro has a couple of features that differentiate it from others. First, I like how it integrates news and blogs together. RSS Micro also shows you searches related to the one you entered. It will also soon give you the ability to subscribe to a blog or news feed from inside a search. The site also retains your search history, but through cookies not user registration.
One downside. Using Ajax means I can’t bookmark individual searches as they all have the same URL (ie. rssmicro.com).