Software

Computing the Distance between Objects
An implementation of the Gilbert, Johnson, and Keerthi (GJK) algorithm for tracking the distance between a pair of convex polyhedra. Source available for nonprofit use.
http://www.comlab.ox.ac.uk/people/stephen.cameron/distances/

I_COLLIDE
An interactive and exact collision-detection library for convex polyhedra. Includes the source code (free for noncommercial use), relevant papers, and an MPEG demonstration.
http://www.cs.unc.edu/~geom/I_COLLIDE/

RAPID: Robust and Accurate Polygon Interference Detection
A numerically robust library supporting general polygon soups, based on oriented bounding box (OBB) trees. The source is available for noncommercial use via email. A paper on OBB trees is available for download.
http://www.cs.unc.edu/~geom/OBB/OBBT.html

SOLID: Software Library for Interference Detection
LGPL standard C++ library for interactive collision detection, based on axis-aligned bounding box trees.
http://www.win.tue.nl/~gino/solid/

V-Clip Collision Detection Library
An implementation of the Lin-Canny algorithm for computing the distance between two closed polyhedra. Source available for noncommercial use.
http://www.merl.com/projects/vclip/

V-Collide
A collision detection library for general polygon soups. Combines I_COLLIDE's "sweep and prune" algorithm with RAPID. Source available for noncommercial use.
http://www.cs.unc.edu/~geom/V_COLLIDE/