Precipitate is a desktop client that ties into Google's Desktop for Mac. (Sorry, Windows user, but this product is only for Apple machines at the moment.) According to The Official Google Mac Blog, "After you install Precipitate, you can use Google Desktop or Spotlight to find files online (such as those in your Google Docs list) just as you would find files stored on your Mac. Then, you can open them in your browser with a single click. Once Precipitate is set up, it stays out of your way, regularly getting the latest contents of your documents automatically, so you'll never need to think about it -- or where you store your information -- again." Precipitate works by creating files on your machine that are imported by Spotlight, then periodically checking in with the server and updating the local files to reflect any changes. I downloaded and installed the client. Truth-to-tell, it took forever to catalog everything the first time I launched it. That's what I get for storing thousands of files online, I suppose.
After it finished cataloging my online files, it was as easy as opening Precipitate and keying in a search. I tried searching for something that I know is both online and on my machine. It showed me all the results for both and where things were. Neat-o.