Treev is a Chrome extension which can search your Dropbox, Google Drive, Box and Trello accounts from a single bar.
To get started you must first add your various accounts to treev, and give it a few moments to read their files.
Once that's done, clicking the treev button (or pressing Alt+S) at any website displays a search bar. Begin to type the name of a file and matches appear underneath. Click the right one - or press enter if it's selected already - and that file opens in the appropriate service.
Treev can also be used as a launcher for your cloud apps. Type "b" as your first letter, say, and "Basecamp", "Bitbucket", "Buffer" and "Box" appear along with any files matched.
As a bonus, Gmail integration makes it much easier to attach files to a message. Click the treev icon in the Gmail Compose window, search for and select a cloud file as normal, and it's immediately added as an attachment.
Treev really needs to support more services, but this is a good start, and the developer says more cloud tools will be added soon.