(A little late but) OAuth has won CNET’s 2009 Webware 100 award in the Editors’ Choice Most Important Technology category:
Why do we need it? Best reason that makes it clear to almost everyone: Twitter apps. Currently, when you’re using a third-party Twitter application, like Tweetdeck for example, you have to give the app your Twitter credentials–user name and password. That’s a key to your entire Twitter account. An app like Tweetdeck could, if hacked or written maliciously, log in to your Twitter account and mess up your account, locking you out or worse. OAuth allows permissions to be set between services, so you could tell Twitter that an app like Tweetdeck could send messages on your behalf, but do nothing else.
OAuth is conceptually related with OpenID (another Webware 100 winner), which allows users to use one log-in to access several services. But it’s quite a different thing. It enables a user’s legion of Web services to work with each other on his or her behalf, even when they are not logged in.