iOS: Mingle is a contacts app that takes some cues from Drafts and Launch Center Pro. Unlike a traditional contacts app that gives you a list and some information, Mingle allows you to launch actions in other apps directly from your contacts list.
With Mingle, your contacts and you apps are tied together. So, if you have a contact with an Instagram profile, a YouTube page, a Facebook page and a WhatsApp number, you can set up Mingle so that you can instantly access that information. For example, I can set a contact's Twitter account and then write a tweet to them directly freom Mingle's interface. Likewise, I can create a calendar event in Fantastical from Mingle.
It takes a bit of time to set up Mingle so that it's actually useful, but once you do it makes finding and contacting people on different social networks easy. It's simple, but Mingle's inter-app communication makes using a contacts app at all a bit more useful.
Mingle ($1.29) [iTunes App Store]