Do you like to play with words? Are you a visual learner? You'll love Snappywords! Snappywords is an interactive, visual, online dictionary/thesaurus. It's great for learning and studying and finding words when you're writing but it's also very fun!
You type in a word and it get a bouncy result showing your word and connections to other words. I typed in "Happy" and got this:
You can get the definition for each word just by mousing over it. The different shaped/colored lines have different meanings which are explained in a key on the page just below the snappyword. In a classroom setting I think this is most useful for exploring the meanings of vocabulary words and I find the connections for scientific words and social studies words to be very interesting and great jumping off points for discussion.
At home with the kids it's just a great way to explore words and study vocabulary except that they have no idea they're studying vocabulary! The Random option is fun - you never know what you'll find. I haven't found anything inappropriate for young children on the site although some of the words and concepts just wouldn't make sense to younger people. My only gripe with the site is that unlike most online dictionaries which give you choices if you enter a word that is spelled almost correctly but not quite, snappy words doesn't respond to "almost" spelling. So if you don't already know how to spell a word you'd have to look it up first.
I highly recommend Snappywords. Have fun!