For a long time I have had a macro which looks up words on dictionary.com. (It copies a highlighted word, goes to the site, pastes it in and hits return.) But recently, their homepage has become so packed with ads and links that it takes simply too long to load.
So I googled for a fast dictionary, and found DEFINR. I don't have much experience with it yet, but it is definitely much faster, loads in about 2 seconds for me, whereas dictionary.com had crept up to around eight seconds. (I've kept adding delay time in the macro so the page would have time to load before the word could be pasted in.)
(Jakob Nielsen says that for the average person, a web page load time of 0.1 second seems instant. One second feels good. Anything over ten seconds is too long. (And especially for a little routine thing you do many times a day.))
Another fast alternative is NinjaWords.com.