Android Thoughts: What Is Your Must-Have Feature?

This summer, on a whim, I decided to write a book. Because I knew it would be a substantial writing project, and because I'd never used Google Docs for anything in particular, I figured I'd try using it for my manuscript. While I eventually ditched it (That's a whole different post), While I was using it, I found that I'd need to be connected to the Internet while writing to have it auto save (duh...). The problem was, sometimes inspiration would hit me on the train home, and even though this is 2010, Metro North Railroad still doesn't have WiFi on it's trains. So I used my Android phone's built in Tethering support. When I got an iPad in July, I also found the tethering indispensable - allowing me to surf the web on my way home when not being annoyed by throngs of Yankees fans on their way back from a game.
For me, Froyo's built-in tethering is the Must-Have feature of my Nexus One. It is something that, now that I have, I never want to be without. What is your essential Android feature?