Waze. It's a sat nav, but has real time traffic alerts, road closures, speed cameras, police trap alerts, etc. It is updated by the users and is absolutely wonderful. I use it all the time.
Happy Cow helps you to find vegetarian/vegan places to eat anywhere in the world.
IMDB for when I'm watching a film and constantly recognising people but can't remember where from.
Star Walk 2 for when I see a cool star and want to find out what it is. Or if I see a planet and want to check which one.