The Wordnik online dictionary is a useful dictionary with an array of tools for word lovers and an API for developers.
The first thing you see when you go to the Wordnik site is words. Words slowly floating down the screen, in various colors, and if you put mouse on them you get a note that says that the word has been favorited, or added to a list, or was just looked up, and this is a good introduction to Wordnik, because Wordnik is all about the words. Or as they put it “Wordnik is all the words, and everything about them.”
An online dictionary can be just a searchable list of words and their definitions. Wordnik goes further. They believe it's best to see words in context, so you don't get just a definition, you get examples mined from a huge corpus, you get tweets. Words can be put in lists, and that's yet another kind of context right there, and they can be tagged.
If you look up a word that it has no definition for, it will still try to find as much as it can about it, in its examples, in definitions for other words, and in the other resources it uses.
Altogether a collection of features that would tempt any word lover.
In addition to those features it also offers a Word of the Day, that you can follow either directly on the site, by following them on Twitter, by becoming a fan on their Facebook fan page, or by a WotD widget that you can put on your own site/blog/etc (the widget is shown at the end of this post).
Another significant feature is the Wordnik API. An API is a an interface for developers. It lets other developers build their own applications using Wordnik's data – that is the words, definitions, word lists, etc. The featured applications page already exhibits a nice collection of applications, including games, smartphone apps, and some which are simply useful.