Newton Dynamics
Julio Jerez's physics engine, which chose accuracy over speed at a time when everyone else was doing the opposite.
While Havok and later PhysX optimised for lots of objects and approximate answers, Newton Dynamics put its weight on stable stacking, precise contacts and deterministic results. It was free but closed for years before being released as open source under the zlib licence. It never won the big studio contracts, but found a following among smaller teams and in tools where the simulation has to behave identically every time.
Its best-known users are Frictional Games, whose HPL engine in Penumbra and Amnesia let players drag doors, drawers and levers with the mouse — horror built on objects behaving believably.