Bullet Physics Library is an open source project with help from the community at the Physics Forum
See the forum at http://bulletphysics.com

The project was started by Erwin Coumans

Following people contributed to Bullet
(random order, please let us know on the forum if your name should be in this list)

Gino van den Bergen: LinearMath classes
Christer Ericson: parts of the voronoi simplex solver
Simon Hobbs: 3d axis sweep and prune, Extras/SATCollision, separating axis theorem + SIMD code
Dirk Gregorius: generic D6 constraint
Erin Catto: accumulated impulse in sequential impulse
Nathanael Presson: EPA penetration depth calculation
Francisco Leon: GIMPACT Concave Concave collision
Joerg Henrichs: make buildsystem (work in progress)
Eric Sunshine: jam + msvcgen buildsystem
Steve Baker: GPU physics and general implementation improvements
Jay Lee: Double precision support
KleMiX, aka Vsevolod Klementjev, managed version, rewritten in C# for XNA
Erwin Coumans: most other source code