I love techniques that are simple but give an interesting look straight out of the camera. For some time I have admired shots taken by Ryan Brenizer where he gets the background to go blue. The technique is no mystery and like many of these processes very simple. The principle is that your flash lights the subject which should render the correct whitebalance but other light in the scene should look blue. You need to gel your flash with CTO gel, then set your whitebalance to match the gelled flash. With a single layer of CTO this would be the tungsten setting but with a double layer you will need to go bluer still. You can either do this by dialing down the K or by using a custom whitebalance. I use an expodisc to obtain a custom whitebalance but you could easily use a grey card.
I have experimented at weddings with mixed results - it can work quite well in a marguee as it makes what can sometimes be yellow walls look a bluey white. The example below is a candid from a recent wedding. Without this technique the background would have looked yellow. For this example the flash was on camera.
I also experimented on a recent test shoot where we had stained glass windows as a back drop. My main light was in a small softbox with a couple of layers of CTO. The window would have looked great anyway but by using this method it matched the blue strands in the model's hair.
This is also a great way of turning a grey sky blue :-)