what are some potential alternative uses for bitcoin mining rigs? could the bitcoin network be used for anything other than processing payments? What other sorts of information can be included in the blockchain?
They're not useful for much besides mining. ASICs that are obsolete for mining Bitcoin could be used to mine SHA-256 altcoins. You could mine and hold speculatively, or just dump for BTC as rewards come in. You can find the most profitable coins
here.
As someone mentioned, they give off a lot of heat. I know someone in a cold climate who lowers his heating costs with his miners. That helps to subsidize his mining electricity costs.