The Daily Buzzword is a reliable page from Merriam-Webster, and RoboBee and BigBot are challenging definition games.
Here's a quick quiz on which reference materials to use and when.
A decent template for dictionary guidewords.
Quite a good crossword puzzle site...here's one on The Beatles.
Try this link when you need a definition but aren't 100 percent sure how to spell the word you're looking for.
