This is some kind of shield bug (I think), also known as a stink bug. He turned up on some sage I picked for roasting tomatoes. I didn't disturb him enough to make him stink, but took him outside and shook him off. If he is indeed a shield bug, he's not particularly bad for the garden, so that's OK.
Oh, and supposedly shield bugs are also edible. Should I have put him in the oven?
Let me know if I'm wrong on my ID! (You can click on the picture to see him bigger.)