Libquantum is a C library quantum mechanics simulator originally focused on virtual quantum computers. It is licensed under the GNU GPL. It was a part of SPEC 2006. The latest version is stated to be v1.1.1 (Jan 2013) on the mailing list, but on the website there is only v0.9.1 from 2007.[1]
An author of libquantum, Hendrik Weimer, has published a paper in Nature about using Rydberg atoms for universal quantum simulation with colleagues, using his own work.[2]