I went with AS Hanging.
They offer a wide variety of solutions: clear nylon cables, steel cables, even poles.
I chose their "Click Rail" system, because it is easy to install and the track presents a low profile.
I also chose their "Secure Aluminum Fame Hooks" as these attach directly to my aluminum frames. Once the picture is hanging, it stays level. AS Hanging was the only supplier I found that had these hooks.
I did a little bit of investigating before deciding on AS Hanging. It seems that there are a lot of choices, and most companies offer similar products. I even suspect that there is some interchangeability, and that you can use hooks from one company with the cable from another.
You need to decide how much weight your system needs to be able to hold. The smaller profile tracks have lower weight limits, but are less noticeable as a distraction. The heavier duty tracks tend to stand out more, and may not be the right look for your room.
There are a lot of choices when it comes to hooks. Every system seems to have both cheap and expensive hooks.
I went with the Aluminum Frame Hooks because they attach directly to the frame. All of my frames are the same size, and they are hung in a grid pattern. I run two lines for each column, one along the right edge of the frames and one along the left. In this way I don't worry about the frames being bumped out of level.
I went with the "Perlon" cables (It looks like thick nylon monofilament fishing line). They tell you that these cables "disappear", but they don't if you have hard lighting on them. Shine a light at your wall, the Perlon cables cast noticeable shadows.