Quick and simple to make, thought I could make some money on referral fees, but went nowhere.
Failure, I suppose: browsers can do the same thing, with a little setup; and, more importantly, people seem to have a preference to navigate to each of these sites and search them directly.
nobody searches with capital letters, so hitting the shift key to make a ":" is a pretty big cost. Just use "g <search>", and detect prefixes you know; it's much faster and less awkward to use.
Would be fairly good to be able to search Wikipedia by typing w: in the URL bar.
Preferences -> Basics -> Manage (Next to Default Search)