Chain hops off cassette sprockets when I stand-pedal
You've got a 'stretched' chain that needs to be replaced.

If the chain has been in this condition for some time, and I suspect it has, I always recommend replacing the cassette at the same time. The reason being that a worn chain wears down the cassette and a new, unworn chain, will fit the old worn cassette poorly and the skipping will continue. I had to learn this the hard way, with multiple trips to the bike shop.

As usual, Sheldon has already laid the issue out in depth:
http://www.sheldonbrown.com/chains.html#wear