Bloglines is a web-based aggregator that I am trying out. So far I am quite pleased: it has a simple, clean interface, and a speedy response time. One nice feature is that it allows you to both import and export via opml files, so you can use it when away from your regular computer, even if you prefer another aggregator for your desktop. Bloglines describes itself as:
Bloglines is a server-based RSS aggregation system. Many blogs and newsfeed services publish RSS feeds, special files containing the content of the site formatted for easy parsing. Bloglines allows people to subscribe to these feeds. Once subscribed, Bloglines periodically checks the feed for changes or additions.