Brian Livingston at IT Management has published the second of his three-part series on web feed aggregators, Picking the Best Online RSS Reader, which compares Bloglines, My Yahoo and NewsGator Online. These were the three most popular web-based readers from his article last week. The review focused on their unique features, and gave NewsGator the best overall ranking for its unique features and significant integration with multiple ways to synchronize with the reading data. (Other readers have the synchronization capability, but NewsGator seems to have the most extensive collection of this.)
Beyond that it would be nice to see more readers discussed, I was most surprised to read that the Bloglines and NewsGator subscription numbers are not completely accurate: it appears neither of them filter out unused accounts, whereas MyYahoo claims to only count those who've used the service in the last month.
Here is the Wikipedia List of News Aggregators, for those who want a complete reference.