iOS: Managing contacts on iOS is not a particularly pleasant experience, and organising any kind of group chat over email or text is a chore. Interact is a contacts manager that simplifies both of those things. Interact works directly with the built-in iOS Contacts app, which means you don't need to sign up for some third-party service or upload your contacts anywhere you don't want them. The main goal of the app is to make organising groups easy, and that's the main highlight. With just a couple of taps, you can create a group, then send off an email or SMS to that group. Adding in attachments is just as easy and Share Extension makes it so you can share files from a variety of different apps with just a couple taps.
Managing those contacts is also a lot faster than the clunky old Contacts app. You can easily delete a contact or add it to a group without muddling around in menus. Creating contacts goes through the Scratchpad, which uses natural language parsing to pull out names, emails, phone numbers and whatever else out of plaintext so you don't have to mess around with fields. Interact is absolutely made for power users and doesn't have much appeal to anyone else, but if you have a ton of contacts and use the Contacts app a lot, Interact makes managing it all a lot easier.
Interact ($7.99) [iTunes App Store]