Phrays is a great way to learn new words and use them in sentences
I love learning new words. However, when I just read what a word means, more often than not I soon forget the definition. If I use the word in a number of sentences, though, it's easier for me to retain it – especially if the sentences are meaningful.
Phrays is a nifty little Web application built on this exact principle. You're supposed to go there once a day and check out the word of the day. It's always a "special" word, but the definition appears right along with it. Then, you click in the text area right under the definition and make up a sentence with that word.
Don't just make up any sentence, though. People will later be able to vote for your sentence, and if it's good enough, it will then appear in the gallery of past winning sentences. Right now, the "winning sentences" are sentences that got just one or two votes, because the tool is in its infancy (and because voting requires an account – boo!). But as more people discover Phrays (and the requirement for an account is dropped, hint hint), I suspect that voting will become much more interesting. As it is, it's a fun way to learn a new word every day!