Makes it easy for your site to use Facebook Connect, in a wholly modular way.
And I've tried at least 6 or 7. This one lets me post to the correct "fan" page with pictures easily instead of posting it to my personal profile, and it's not horribly broken. The few other features I've tried are very basic yet functional, but I haven't tried them out much as it was the publishing aspect I was most concerned with.
Re: the Facebook app portion, the only thing that I see that Otto neglected to mention is that you have to register your profile as a Facebook developer if you haven't already. Anyone with a FB account is allowed to do it, it's simple to do and there's a large amount of documentation online. Once you've registered, and you click create app, the info you need and the few fields you need to fill in are all on the first page (basic).