Virtual Mars Habitat
Explore Mars Now
Interactive Flash interface for exploring a hypothetical Mars habitat. I think it veers more towards sci-fi than hard science, but the design has been thought out in great detail, plus it looks good too.
(via Mars Rover Blog.)
Mars Simulation Project
The Mars Simulation Project is a free software Java project to create a simulation of future human settlement of Mars.
The simulation is a multi-agent artificial society set in a detailed virtual world.
XML configuration files allow the user to modify the simulation properties.