Well, Alexa has a ‘related links’ tool if you search for a particular website, but I searched up Fluther (.com) and it was surprisingly inaccurate. It lists Digg, Twitter, and this site called Betamarker… I also looked up Design Bump (which is a Digg-like site, but geared towards designers on the web) and it was marginally more accurate; it listed sites that also deal with design, but many are blogs and not similar design-related social media sites. I believe websites that offer some sort of similar-site thing tend to do a poor job…either because they collect related URLs in an automated way that works out badly, or they have a user-rating system that doesn’t produce accurate results because there aren’t enough people using it/