I am (or have been) involved in the following open-source projects:

 ALeSiA

A tool for automated performance experiment planning and incremental algorithm selection (Scala).

 SESSL

A domain-specific language for simulation experiments (Scala).

 JAMES II

A Java-based Framework for Modeling & Simulation. I'm linking my archived repo of the (public parts of the) project here, as the original domain (jamesii.org) is no longer in use.

 P3J

A Java-based tool for probabilistic population projections, built on top of JAMES II.