ReadWriteWeb

Lingro is a fantastic collaborative dictionary and translation service that combines open dictionaries on the web with user contributions under a CreativeCommons license. It's particularly useful for people reading in a second language who just need help with a few unusual words now and again. Anyone will find the basic two-click dictionary lookup feature useful though.

The service is very feature rich, but the easiest way to use it is with a javascript bookmarklet. One click of the bookmarklet makes every word on the page you're on clickable for an instant dictionary pop-up. You can add your own definitions right in the same pop-up. For many of the words there are audio pronunciations provided, too.