Word on the web is that www.Wordie.org is the latest geek addiction.
You won’t necessarily build up your lexicon because there are no definitions there, only links to definitions. But you can keep track of those words that you hear or read and know you’ll want to use again like troglodyte or gouache. When you look up new words, you see links to various definitions, as well as a list of other users who have marked that word to remember. It’s silly but somehow addicting in the way Flickr is addicting.
This is great for word-lovers like me.
Try it for yourself!