Roland Hess is an artist, author, coder and Blender enthusiast. He is also the creator of Blender People, a crowd simulation add-on for Blender. It is still under development and requires that you have MySQL installed on your computer. I haven’t attempted to get it working on my machine yet.

A more expensive solution to the crowd simulation problem (and more fully implemented) is Massive [Wikipedia article]. From the Massive website:

Massive is the premier 3D animation system for generating crowd-related visual effects for film and television. Using Massive, an animator or TD designs characters with a set of reactions to what is going on around them.

The reactions of the characters determine what they do and how they do it. Their reactions can even simulate emotive qualities such as bravery, weariness, or joy. The agent reactions can control key-framed or motion captured animation clips called actions.

Characters that perform on their own in this way are referred to as agents. Massive is a system for designing and running such agents. When scaled up into the hundreds – or hundreds of thousands – the interaction within the crowd that emerges from these individuals is highly realistic.

By building variation into an agent, the Massive artist can populate a scene with individuals who are unique in everything from physical appearance to how they respond.

Massive costs between $6,000 and $18,000 USD. Whew.