I heard about Rojo for the first time just last night, and it appears they just launched their public beta today. I just created an account and I've only spent a few minutes tinkering with it, but I think it's got the best UI of all the web-based aggregators I've seen. The design is clear and simple, but functional and fairly intuitive.
Only major complaint I have is that, for a service being rolled out today, it uses surprisingly little (i.e. none, as far as I can tell) Ajax/DHTML (for example, there's no excuse to reload a page simply to add a tag to a feed).
Speaking of web-based aggregators, a friend of mine wrote a prototype XSLT front-end to IlohaMail's feed aggregator, which takes advantage of the fact that IlohaMail can re-publish aggregated feeds as a single Atom feed.
