Warmshowers.org is my biggest site, with more than 7500 members, and almost certainly the most useful. It's a reciprocal hospitality site for touring cyclists - people offer touring cyclists a place to stay and a shower. It's been in English for years, but that cuts out a significant part of the world's population, and it means that options for cyclists in Latin America and several other places in the world have been more limited than they ought to be.
But we just launched es.warmshowers.org, a Drupal 6 version with all the key parts internationalized and then localized into Spanish. We're very proud of this accomplishment!
It took a lot of work by a lot of people to do this. First, Chris Russo nearly singlehandedly upgraded the site to Drupal 6, a significant accomplishment because of the custom modules we have developed for the site. Then we began learning how to do all of the localization into Spanish, and learning how to get a group together to do the translation.
There are probably four parts to the translation of the site: