ARToolKit http://artoolkit.org/ now can be used for free in commercial applications under LGPL v3.0. When working on James Burkes PhD, which focused on AR for games based upper arm stroke rehab, we found that ARToolKit was easily the best tool (and was free for academic use). After completion of the PhD we looked into commercialising the games but the cost of the commercial license was a real problem going forward with this. The new license terms are obviously very helpful for people wanting to harness state-of-the-art AR technologies within their software and Apps.
ARToolKit now available under LGPL v3.0 for free use