Erlang is currently getting a lot of attention as a language that supports concurrent programming with a large number of simultaneous lightweight processes (or threads if you prefer). But there are few reasons its ~sibling - Stackless Python - should get less attention, some preliminary benchmarks support that.
During grad school some students and I used Stackless Python in combination with MPI to create a simple simulation of a large number of players and NPCs in multiplayer games on a cluster. The game world was divided onto machines in the cluster and on each machine a few thousand tasklets (also called microthreads) each representing individual players and NPCs ran. When they ran into each other they communicated using channels. When players or NPCs came to the boundary of the game world we sent the serialized state of the tasklet representing the player or NPC using MPI messages to the neighbor CPU world.
Stackless Python turned out to be a very nice language to code in, and in fact the students and I tried to start a company providing microsimulation consulting services using it. Unfortunately such services turned out to be of much less demand than anticipated so we had to stop our efforts in that direction. But that wasn't Stackless' fault, in fact I would recommend having a look at Stackless Python if you consider doing microsimulation.