A new online dictionary and thesaurus launches today called WordSource. It’s main benefit over sites like Dictionary.com is the fact that the site is very cleanly designed and contains no advertisements. You can also look up a word by simply adding it to the URL (so word.sc/example will pull up the definition of “example”).
The site also has a number of social features, allowing users to upload photos that are relevant to the word, add tags, rate words, etc. Seems like a bit of overkill to bolt a social network onto an online dictionary, but there you go. If they hadn’t built it, someone else probably would have.
I tend to use Google to define words (just type your query as “define: example“), and many people use this Firefox plugin as well.