What are your favorite iPhone apps?

Now that I've downloaded really important applications like a timer for cooking, a program for playing with molecular structures, and an app that lets me listen to NPR on my phone, I'm ready for more. I'm seriously overwhelmed by the iTunes App store. I found a program for learning human anatomy, a laboratory calculator for making solutions, music programs, wow! Who knew? I know some schools are using iPhones for teaching, but I never realized how much was out there. Deepak recommends something called TouchPhysics. What are you using? What cool apps do you suggest?