The folks at QuietWrite propose a better way to do sign-up forms: don't.
Basically, we eschew the typical signup wall for a more gradual process over a longer period of time. As the user plays with our product, we gently prod them at certain checkpoints to give us more info. This results in a better and more natural user experience.
Of course, this is nothing new, and Posterous has been a, dare I say it, poster-child of this no-signup or low-touch-signup movement. Every application benefits from allowing the user to stick themselves in as far as possible before being asked to fill in a details form.
This must be carefully balanced against your business's needs, however. Perhaps your business depends on leads capture to such an extent that it needs to capture emails immediately or lose the customer forever. In my experience, those aren't the most thrilling of businesses, but still, this is another decision that you should make deliberately, not by default.