TweeFind is one of the simplest interfaces (do not confuse this with Tweepfind we highlight right here). Almost too simple unless you know what it is there for. Enter a search term and get results back by the ranking of the user, instead of just random tweets.
You can search just English tweets or all tweets and the results. You get a list back with some gray circles under the usernames. They fill in gold, I guess, as the user ranking is better. I have no clue how they decide user ranking though.
You can then tweet that search back to the stream. A clean interface and they sneak in an ad right above the returned results for revenue. Not a bad plan.