I did hot water followed by 10 minutes of ice cold (a la 4 Hour Body mentioned above) for a while. I found I needed to set a timer, otherwise I would think it had been 10 minutes when it had only been one. I did find it to be invigorating, and felt like a million bucks every time it was over. However, after a few weeks I started dreading getting under that cold water every shower, and eventually stopped.
I am not sure if it actually had any beneficial effects aside from waking me up, as those things are hard to quantify. I think it should be said that it may increase testosterone. I do not think the science on that is definitive. It also may improve immune function, but I think that is another uncertainty.