What's your one favorite app you want everybody to know about?

There's a very good chance this has been done before, but what the hell.

Mine: Any.DO

I tried plenty of task apps. This is by far the most beautiful. The design has remained the same for a while now, and for once, it's a good thing. Since they've added a brilliant new Chrome extension, it's the most practical I've used as well.

What's your favorite app?