My mom emailed me a link to this site which not only helps you improve your vocabulary, but also helps alleviate world hunger.
Free Rice donates 20 grains of rice through the UN World Food Program for every word you correctly define on their word quiz.
The cool thing (to me, because I am a geek) about this word quiz thing is that it adjusts to your vocabulary level, giving you progressively harder and more obscure words. There are a total of 55 levels, and the highest I have gotten is to 51. And that mostly because it is multiple choice, and I made some good guesses.
Some of these words are hard to work into conversation, though. I might could work in "lagniappe" (small gift) for example "I have a butterfly pocket mirror to give as a lagniappe to the first person who buys a skirt from me", but then there are words like "catoptric" which is apparently, "mirror-related". I'm having a hard time coming up with a situation in which that would be useful. "I have a catoptric langiappe for the first person who buys a skirt from me"? Or words that could be used, but you would sound pretentious - "lanuginous" instead of "downy", "apiculate" instead of "pointy". I'm guessing a lot of them are actually words that would be used in a scientific context. My spell-checker, by the way, is highlighting all of these. But some are just fun - "katzenjammer", for example, which if I remember right, means "hangover", although it sounds to me like it should mean the night that led up to the hangover.