This. I am continually amazed at how few people use Audible. It is an amazing and rich source for books recorded in very high quality (read by voice talents, no stops during audio). I finish a book every 2-3 weeks just by listening on my way to and from class.

Grooveshark is another service I use my phone almost daily for. Play virtually any song on your phone.

Myfitnesspal is yet another. Record daily caloric intake and exercise easily (since you always have a phone around and since you don't know all the calories in the food you eat, but a search-enabled website does).

I also play backgammon while waiting for the bus.

I never take maps to unknown destinations any more. I still remember the times we printed maps before going to tourist destinations in nyc.

I check my email.

I check my calendar.

I talk on skype (via skype mobile). I do international calls without being charged international rates.

I check the weather (mostly the temperature) to know how heavy to wear.

I occasionally take photos when I need to record something (on a blackboard, for example) but don't want to write it down.