A website, developed by the makers of the Oxford English Dictionary, devoted to saving words that are rarely used and therefore risk losing a place in the dictionary.
This fun site, one vision of a social media-enabled dictionary, offers not just definitions of words (from a variety of sources), but also contextual references, and the ability to tag and/or build a community around a given word. It's quite hard to describe; I recommend clicking through a few words as they float by on the home page.