Definr

Definr is the cleanest, most simple, and definitely fastest online dictionary I've come across. I don't think it's very widely used yet (probably because it's somewhat new and because people may not trust it since it doesn't have "Webster" or "Oxford" written across the top of its page), but it's a great tool if you ever use an online dictionary. Definr.com is based on Princeton's open WordNet 2.0. They have a few different ways to look up words including a Firefox extension (my favorite), which has a really good auto-suggest feature.

To get an idea of how unknown Definr.com is, a search of "definr" on all three big search engines (Google, Yahoo, Live) will not return links to Definr's actual site, which I thought was kind of amusing. I hope Definr doesn't change a thing as it gains more users because it's rare to come across a tool that's near perfect through its simplicity.