Real-time crowd simulation requires fast, realistic methods for path planning as well as obstacle avoidance. The movie below details from EPFL Virtual Reality Lab, Switzerland's Motion Planning Approach:
Navigation Graphs allow efficient navigation planning and simulation for crowds. They can be automatically computed from the mesh of the environment where the crowd is to be set. In real-time, EPFL's solution is able to simulate and render tens of thousands of people.
Take a look at EPFL's website for more information, papers and movies.