Best App Nobody Knows About?

What is your favorite app that the rest of us probably haven't discovered yet?

My favorite is called Yik Yak. The app was designed by a couple young guys at a college, and campuses are where the app is most popular. Yik Yak serves as an anonymous community message board where you can post anything you'd like for people to see within a few mile radius. Other users can vote up or down your Yak based on whether or not they like it. If you receive 5 "down likes" your comment will be deleted.

WARNING: Yik Yak features a TON of stupid posts, and at times inappropriate material. If used correctly, it can be a helpful and informative tool.