How do I capture emails with Facebook fanpages?

Adding an opt in form to your Facebook page is relatively simple and there are many ways to do it. Aweber and mailchimp both ahave easy options to add your signup form directly to a Facebook page via their Facebook apps but that's a pretty basic way of doing things and doesn't really give anyone a reason to actually opt in.

A better way of doing this is to create either a competition or squeeze page within Facebook. If you're doing this from scratch by yourself then you do need to be aware of the promotional guidelines as the previous poster suggested, however there are plenty of tools out there that take care of this side of things.

As the creator of one such tool, i can't speak for the others, but FanBoom provides lots of ways to build a list from your Facebook presence. Running a competition typically produces the best opt-in rates but creating a traditional squeeze page also works. Using FB connect is a third option possible with FanBoom which subscribes users to your list with their Facebook email addresses without them having to even enter their name/email address.
Lots of options really, but converting Facebook fans into subscribers is a very powerful strategy, and by using Facebook's viral capabilities to get subscribers actually sharing your signup form with their friends, very explosive list growth can be achieved from minimal effort.