Researchers at Microsoft and an intern from Carnegie Mellon University have built "telepathwords," a service that helps you determine whether or not your password is weak by predicting subsequent characters as you begin typing it. With each character you type, the site will reveal what the next one's most likely to be. And if it's correct, you should probably start thinking up a key that's a little more complex.
The site uses information derived from common passwords, phrases and password selection behaviors (adjacent keys, for example.) in trying to predict each character. And while it says it can't necessarily prevent weak passwords, since those are still predictable to hackers that know you or how you choose your passwords better than it can, it's still a fantastic tool for when you're picking your next login code. The site also maintains a log of mouse movements and how long you took to type or delete characters, for "research." Sure, Microsoft.