There are so many apps out there for the iPhone, Android and other phone platforms, but so few essential apps. What's the one app that you just couldn't live without?
For me, it's Instapaper. I use it virtually every single time I ride the subway, and it's allowed me to read so many amazing articles that I just don't have time to read online during the workday. And with built-in sharing options and a great editors picks list (combined with the wonderful Longform.org), it provides me with a neverending supply of great content so I always have something to read on the train. The only downside is that it's kept me from carrying a book with me, as that's just one more thing to carry.