Because I have a big love for my smartphone (currently an iPhone 3GS) I am always curious about what apps people have running on their phones. I haven't drunk the apple kool-aid and its entirely possible that my next phone will be something different - So my question to you is; What is your favourite app/s on your phone? And why do you like it or what do you use it for?

I'll start:
Google Maps (came free with the phone). I may live in the middle of the flat, flat prairie, but I'm always finding this useful whenever I travel to the big city.
reQuall (I'm using the free version - but am seriously considering the upgrade to the paid subscription). I need to be reminded of things, and reQuall's ability to remind me via email is terrific. It looks like they are releasing an Android version.