For me it would be Skype. To be able to make free web calls online anywhere in the world- even with full streaming video is just insane. Just some years ago it wouldn't have seemed to ever be possible and yet here we are. It's not the most reliable service in the world, I often suffer dropped calls or quality issues but it has done so much for my life overall when it comes to communication.
I don't think that there are any apps that have actually changed my life, but there are a few apps that have made my life easier. For one, Vonage has made it super easy for me to leave free voice messages for my friends and receive free voice message. It also allows us to share pictures with each other.
Google maps. Don't really have to plan that much ahead, you can find what you are looking for easily etc. I love wondering around the place when traveling, so it also helps a lot to find my way back from running around a bit too much.
Google Maps. It's nice getting an address from someone and just punching it into your phone. I used to hate going somewhere with unfamiliar roads because I always felt so lost. Google Maps helps me feel comfortable when I travel far away into unfamiliar territory.
Ah, I happened to forget this one. I think it has changed my life more than Skype actually, being able to so easily get directions is invaluable. There was really nothing like that before Google Maps came along. GPS is also a really cool technology, though I still need to get a really good dedicated device.
