Techcrunch has done a neat review of the following web based RSS readers:

Attensa Online
Bloglines
FeedLounge
Google Reader
Gritwire
News Alloy
NewsGator Online
Pluck Web Edition
Rojo

Summary: For pure performance, Google Reader and FeedLounge are the fastest in the tests.
Bloglines and Rojo are the best choice for a feature rich application (Frank says Rojo blows Bloglines away on ?web 2.0? type features).

I personally use Newsgator online to read RSS feeds along with RSS Bandit to synchronize RSS feeds with my computers.

Techcrunch