Ninja Words is a new site that I just read about on Richard Byrne's excellent blog. NW is an online dictionary which is very easy to use and quite good.
What I like so much about NW is that it tracks a user's history so you can refer back to words. However, you do have the ability to empty the history if you don't want to keep a record of that. It reminds me a lot of Apple's built in dictionary w/out the ability to cross-reference.
Below is my example...
Give Ninja Words a try by clicking here.