NinjaWords is my new favorite online dictionary (seems like a lotta ninjas in the searchosphere lately). It is exactly what it says it is - fast. Can look up many words - at once. No ads. Can be added to your sys tray (windows), dashboard (mac) and Firefox search bar. Check out the about page to see the sources of the definitions (cool!), and read the author's interesting blog post about why he created it. I like it.
You can find this again in libraryland on the search engines page.