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.