For those of you with an iPhone/iTouch: What has proven to be the most useful or most used app that you have downloaded?

I am finally on the “i” bandwagon since I got an iTouch a few days ago. I have had a quick look through but have so far only downloaded one app. What are some of your favourite apps? Humorous, useful or just good boredom killes, what would you recommend this “i” newbie?