Let me be the latest to sing the praises of the Blackrapid R-Strap. I got interested in the strap by reading about it on David duChemin's blog — he's sponsored by Blackrapid, and has come to love the R-Strap after dragging it all over the world. Sounded like enough evidence to me. But if you prefer, just Google it and see how many others have had great experiences with it.
The idea behind the strap is that you can easily get the camera up to your eye from your carrying position. A small bracket screws onto the tripod mount (or tripod collar on a larger lens), and attaches to the strap with a small metal clip. The camera hangs upside-down next to your hip or just above your butt cheek. The whole assembly slides along the strap, so all you do when you're ready to shoot is grab your camera by its grip and bring it up to your eye, like a western gunslinger.
Others have done this already, but I made my own "quick draw" video to show you the strap in action:
You could probably make your own R-Strap. The design is simple, yet effective. DIY versions have popped up already. Personally, I choose to trust the craftsmanship of the folks at Blackrapid, who seem to listen well to photographers — already they have a replacement clip assembly that they offer as a $0.01 upgrade (albeit with a separate $4.99 shipping charge).
Believe me, or believe everyone else - this is a great strap that'll free up you hands when you're walking, hiking, or using two camera bodies at a wedding or other event.