Ask HN: Good side projects for getting internship?

It's a pretty common fact that side projects are a good way to impress recruiters at large software companies. I'd like to take that a step further. What makes a side project particularly attractive from a recruiters point of view? Is it more about content or simply how you manage to sell it to recruiters?

(Forgive me if this is against HN protocol, but any unrelated advice on getting internships would be nice too)

Thanks.