This is similar to the Facebook Object Debugger. It lets you preview your twitter summary card when you paste in a link. As someone running a website, you can set it up so that there is a title, summary and an image. If you want twitter to support cards with your website, you do have to set it up on twitter’s developer site.
Most of the things that Brett Terpstra does are a little over my head, all of which tend to increase productivity in a magical way. There are a few projects that anyone can make use of. SearchLink is one of them. I just started using it. It makes generating markdown links so simple and quick. It is definitely worth checking out.