Lowepro SlingShot, which comes in several sizes.
For me, the best part of this bag is that I don't need to stoop down to put my camera away. And that allows me to go without a strap, which is nice if I'm mostly walking around and taking the occasional picture. I only use a strap about half the time I'm shooting.
For a 5D2/5D3 + 2 small/mid lenses, this bag is great. But I've found that three midsize L-series lenses + 5D3 gets a bit heavy. The sling design is great for light loads on short walks, but not recommended at all for heavier loads on all-day walks or hikes.