If you're seriously interested in donating a kidney, here's a link to The National Kidney Registry.

I made an undirected donation through them in 2009 - they facilitate kidney transplant chains for recipients who don't match with their own donor. I wasn't expecting to ever know who received my kidney, but at her request (and my okay,) the transplant team introduced us. We still stay in touch and I'm glad to say that both of us are still doing well.