Good back up/second body

Hello
The title says it all, what do you think is the best backup/second body to have in your bag.
I am just curious to see what responses would be, if needed I can make this a poll.

Are we truly talking backup? Or do you want a second body to carry simultaneously? Because a backup can be any cheap old body, but a secondary body might be easier to use if it is similar to your primary.

I think that it helps if the second body is from the same, or similar, model range as the primary. That way you can use the same batteries and the controls are the same.

Here's a vote for the 7D. I'm pretty happy with mine as a backup/second body. I also use the 7D for motorsports photography, and after a bit of a learning curve with the focus-related C.Fn's on the camera, I'm now pretty happy with it. The 7D is nice as a second body because it's tiny and lightweight (which actually kinda sucks when paired with an L lens), and because it has good features and performance for the money.

I think a properly-working 1DM3 would be a better primary camera (than the 7D), but as a backup, the 7D is a great choice.