RePast A popular Java-based social complexity simulation toolkit.
Ascape Another popular Java-based social complexity simulation toolkit.
Swarm The venerable Objective-C and TCL-based social complexity simulator, from which RePast and Ascape (and MASON) owe much.
TeamBots A Java-based high-level, 2D abstract robotics simulator and hardware API.
Player/Stage A C++-based (but language-independent) 2D and 3D abstract robotics simulator and hardware API.
Breve A 3D simulation toolkit for MacOS X, Linux, and Windows using an interpreted language called Steve. Very impressive.
StarLogo A simulation toolkit in Logo, ostensibly for educational purposes, but extensible and powerful.
NetLogo Another, somewhat newer member of the Logo simulation family. Very nice!
Processing A beautiful Java/OpenGL environment for simulation, animation, multimedia, and playing around.