Stereo is an old free photogrammetry software, that was developed in 1997 by Paul Sheer during his PhD. At the moment it's one of the few free software programs that do photogrammetry (correct me if I'm wrong), that was due to be integrated into GRASS but is now stuck.
Yesterday I found out that errors I had compiling Stereo from source code were due to using GCC 4.1 (too recent for 10 year old code). Compiling it with GCC 3.4 gave just some warning but now I have it “working”. Of course there's no magic so the interface is still very very buggy and unstable.