Box2D is a 2D physics engine for games.

Features:
Collision
    Continuous collision detection
    Contact callbacks: begin, end, pre-solve, post-solve
    Convex polygons and circles
    Multiple shapes per body
    One-shot contact manifolds
    Dynamic tree broadphase
    Efficient pair management
    Fast broadphase AABB queries
    Collision groups and categories

Physics
    Continuous physics with time of impact solver
    Persistent body-joint-contact graph
    Island solution and sleep management
    Contact, friction, and restitution
    Stable stacking with a linear-time solver
    Revolute, prismatic, distance, pulley, gear, mouse joint,
      and other joint types
    Joint limits, motors, and friction
    Momentum decoupled position correction
    Fairly accurate reaction forces/impulses

System
    Small block and stack allocators
    Centralized tuning parameters
    Highly portable C++ with no use of STL containers