We have a custom application that's nearly unique in our particular community/fandom. There are others like it, but they're all rubbish We get the banners that application produces on 4chan, Reddit, and Facebook quite a lot, and they tend to have a link back.
The problem there, of course, is converting people who use that application into members of the rest of the site. I'm hoping XenForo (and my custom code) helps with that.
Normally by kidnapping the members first born kid and leaving a message in the ransom note to join our forum. Just Kidding.
In my case, it is a Forum that I took over from another person a few weeks ago. There was a small group of members that came with the deal and I am currently working on building the user base. Unfortunately as the niche of the forum is in a fairly densely populated area, it is not easy to build a membership pool. The niche is Web Hosting and Webmaster Discussions.
The last few members came from my annual April Fools Day Joke - http://ServerMeadows.com (Server burrial and cremation services) in which WebHostingChatter.com was featured on a few pages.