For a while I've had a macro which copied a highlighted word and then looked it up on Answers.com.
But recently that site does not place the cursor in the Search field upon opening, breaking my macro (not even hitting tab works). So I have chosen dictionary.com instead, and I must say it works very well. Actually even better, because sometimes Answers.com was too slow to load, also breaking my macro, but it never seems to be the case with Dictionary.
It's way faster and easier than using a paper dictionary. Gotta love computers and the interweb.