Our design, made by Nitto of tubular CrMo steel, then nickel plated for lustrousness and durability. It has a platform for a big load--sleeping bag, mat, tent, whatever--and horizontal bars lower for hooking on regular old panniers. The lower position isn't as low as low-riders, but it's not that important to keep the load that low anyways.

The stressed portions are trussed up (triangulated) to totally prevent failures.  For most tours, you don't need a rack with this much load-carrying capability. But when you do, this is the one to get. 

One size works with 26", 650B, and 700c.