Without God?

We had quite the bru-ha-ha here in our town about a slogan that was put on a bus. What was it?

You Can Be Good Without God.

My question is, if we have no moral compass as outlined by a higher power, then what do you base "good" on?

Without a higher power of some kind, no matter what name you call It, how does one judge what is right and what is wrong?

Enlighten me.