I was browsing the Facebook Developer Wiki over the holidays and came across a neat little feature. It is possible to give users who aren't on Facebook a specially crafted URL that will allow them to sign up with your application already added to their account.

Simply direct non-Facebook users to the following URL to sign up:

http://www.facebook.com/p.php?api_key=xxxx

(Where xxxx is your application's API key)

This feature can come in handy if you have an existing user base outside of Facebook and want to encourage them to sign up to use your application.

See the Developer Wiki for more details on this feature.