I recently needed a very reliable pose estimation system, in which ar_pose (http://wiki.ros.org/ar_pose) was not that good as it was depending on the very basic ARToolKit library.

In order to avoid occlusions, I needed to use boards of markers (you could also use it as a 1x1 marker board), and it could detect virtually unlimited markers.

Moreover, it is able to handle many cameras at once, and finally display the result in the rviz (http://wiki.ros.org/rviz).

I would love if you discover things yourself further, so here is the link:

Please try it and let me know your impression and feedback which is highly appreciated!