Agile: The Elevator Pitch is a good collection of short concise summaries of what "Agile" is. This is my favorite...
Make progress with imperfect information and adjust as new information arrives.
One Day Sprints John Cutler has lots of interesting things to say about Agile Software development. He's worth following.
Agree on something small
Postel's Law Also known as the TCP Robustness Principle:
Be conservative in what you do, be liberal in what you accept from others.
Not just applicable to low-level data transmission, this is a rule to live by.