Something you couldn't leave the house without:
My cell phone.
aka: my "brick"
I probably rely on it too much to be honest. It's my watch, my alarm clock, and my ticket to the social world. I'm pretty sure the only phone numbers I have memorized are my own, my home, and my mom and dad's cell phone numbers. And they are on speed dial anyway, so it's not like I need to remember them...haha