Yup, that's a Garden or Garter snake (depends on where you're from). If he's not too badly off and you happen to ahve caught him, try looking for a wildlife rehabilitator near you. The one near me will actually patch up frogs that have been hit by lawnmowers. He uses fake skin and then eventually releases them once their real skin has healed up under that. It's pretty nifty.

I wish I had more of these, but the raccoons eat them. They clear slugs completely, and the big ones go after voles, happily disappearing into the tunnels (I have on idea how they get back out). If I am on vacation, they move into the garden, but as soon as they see me they are gone.