The concept behind Greplin is pretty simple - they index the various services you use across the cloud, like Gmail, LinkedIn, Dropbox and Twitter, and let you then search through your personal history.
This is how Daniel Gross, the founder of Greplin, hit across the idea, “He was on his way to a party, and he didn’t remember where the address was stored. Was it a Facebook event, or in an email, or in his calendar? It was a pain to try searching all these things from his phone.” I can personally recall a few occasions where I couldn't remember if the conversation happened on Facebook or Gmail, or I bookmarked a link on Reader or Delicious.
After Greplin has indexed the services you've added, you can either search directly through their interface or through the Firefox or Chrome extensions. A search for Sunlight Foundation across my network yields results from messages, events, people, files, streams and links.
The utility of the service is apparent after just one search, personally I'm hoping Greplin has an Android app in the works. For a quick walk-through, check out this video from their team:
If you're interested, you can sign up for an account here.