Those little green reweet buttons you see across the web on sites like this one have helped TweetMeme rise in popularity . The buttons are now so ubiquitous that the service has seemingly become the de-facto retweeting mechanism for content on the web. But it looks like it's about to get a challenger, with a killer name, Retweet.com . Retweet.com currently only has a a landing page saying that it's "coming soon," so it's hard to know exactly what it is from that. But there are plenty of clues around the web pointing to it being a TweetMeme competitor. The main hint comes from a design contest taking place at 99designs. The bids are now over $1,000 to design the site, and all of the mockups look very similar to TweetMeme (which, to be fair, takes a lot of its look from sites like Digg).