StumbleTweets only looks at tweets the last 6 hours that contains links and then sends you right to them. You can limit what you get by using terms such as TwitPic or Blip.Fm to get only those.
Once it finds them you can esily retweet. Basically this takes finding the best retweeted and reverses it to links that you can retweet.
They give samples of trending topics on the homepage to quickly get you informed on what is being talked about on Twitter. My first stumble sent me right to YouTube.