Greplin is a relatively new service that lets you search throughout various online spaces to find anything you might be looking for. Essentially you sign up, and it can then hook into your Twitter, Facebook, Gmail and Google Apps, Basecamp, Dropbox, Evernote etc. accounts and allows you to search through all of them from one place. Plus, they have what is supposedly a truly stellar iOS app.
Need to find a phone number but forgot where it was that you put it? Greplin.
Some random file missing? Greplin.
You can see where this is going and how it can potentially be very useful.
As it happens, I have yet to actually use it — nor can I see me needing it any time in the future, but I can certainly imagine it coming in rather handy at some point down the road. It's definitely something to bookmark (or throw into Yojimbo/Evernote) even if you don't yet see the need to sign up.