I discovered most of these via Gary Price's Best Betas presentation at Internet Librarian. As web apps are exploding and replacing desktop ones, I foresee that services that enable cooperation and aggregation amongst web services will be increasingly in demand. Here's my first taste.
I just discovered Hojoki so I thought I would update this post & include it. Hojoki isn't terribly different from the other cloud aggregators here; you can add accounts, from Google Docs to Mendeley (which is a cool bonus for researchers who use that service) to Github, and all updates appear in one place. You can create "Projects" which function as folders and can share folders with people. Updates from any service appear rapidly in Hojoki's equivalent of a timeline, making it a great real-time collaboration tool. Right now, the list of services supported is moderate but interesting. It's particularly cool to see Github & Beanstalk support, meaning that this could make for a better code-collaboration tool than the others on this list. However, lack of FTP/WebDav support makes it more limited than, say, Otixo, which is still what I would recommend as a singular desktop for all your files in disparate cloud applications.Blekko and DuckDuckGo, which I believe don't alter results based on personal information) other search engines are these days, but in that you give it access to accounts like Gmail, Facebook, Twitter and it indexes the results. The list of services you can index is fairly large. The user interface is minimalist and slick. Overall, it looks well-done and has the largest chance of making it into my everyday Internet usage of anything on this list.
My only gripe with the service thus far is how it orders search results. First comes Mail, then Events, then People, then Files, and finally Streams (Twitter/Facebook accounts...not much different from People, actually). That's almost precisely the opposite order I'd like to see. When I imagine the utility of something like Greplin, there are two basic use cases: "Damn, what was the cool link I saw somewhere but didn't save?" and "Shoot, where is that document I wrote, in Google Docs or Dropbox?" Neither of those use cases involve my Gmail contacts or Calendar events, yet those are the search results that rise to the top to the detriment of more useful items. I figure Greplin is still young and custom result ordering is probably on the way, so I'm not too concerned. But it does point to perhaps a fundamental misconception of what the service is for.
The list of services you can combine is pretty great, though; around the same size as Greplin. I didn't see any WebDav or FTP support like Otixo, though.
Goodreads and Last.fm, simply as repositories of information about myself.