nabbber is probably best described as
the Twitter for language learners.
Like on Twitter you follow other people you are interested in and you share short messages. Whereas on Twitter those messages can be about everything, nabbber is build for the exchange of vocabulary.
nabbber is combining the classic flashcard model with a social networking aspect. You create your flashcards as you normally would on other services but rather than keeping them for your own you share them via a social stream. Other people who are learning the same language as you do can then follow your words and vice versa.
The premise is to share words or phrases you learned with the community and therefore enrich each others vocabulary. Whereas other services give you prefab stacks of 20 words to learn nabbber is based on a live stream of language learners you can follow based on same interests etc.
nabbber is a project by Alexander Zaitsev and has been launched in November 2010. At the moment the community is mostly Russian based but nabbber can be used for any language combination and the interface is in English.