Acumen is currently being developed by Prof. Taha and the rest of the Effective Modeling research group.

Generous funding for Acumen's development is provided by:
We have recently moved development to GIThub.  If you are a member of the development group, you can view bug status reports and report bugs online.  We expect that the next release (2012) will be delivered using GIThub.

Before that, Google Code for bug tracking and as a wiki for development.  You can find the Acumen Google code project web page here.

Challenge problems for things to model in Acumen can be found in many domains, including
And, of course, there are a lot of cool robotic systems already out there that can be improved and studied through modeling and simulation.  Here's a random list of things that have been brought to my attention:

Microsoft has an initiative on modeling the world discussed here and here.

Related activities by other groups and organizations

There are several important projects that related to different aspects of Acumen, including KeYmaera,  AnyLogic,  Frink,  MathScheme.

Related research events related to Acumen include ICCPSCyPhySIMPARECRTSEEOLTUMCPSDEVS/TMSPADSCoMetsAVICPS, and Mod4Sim.  Related summer schools include ESSA.

Related multi-body simulation tools.

Related hybrid systems simulation course at Standford.

Related modeling course at Franklin Olin College.

Related numerical methods are covered by lectures notes available online.

Related physical modeling and graphical methods are covered very well on the home page of Doug James.

Related tools and libraries include MATLAB SimulinkMapleSimMathematicaAutoLevLMS20-simWebots, and many others.

Related research pages exists at many different universities, including at The University of Umeå.

Related textbooks exist on topics such as mechanical systems.

On the education front, there are also languages like Phun that share the same educational goals as Acumen.

Related blogs include Kristoffer Lidström's blog on Cooperative Cars.  Dick Lipton also has a very interesting blog entry on notation.

The NSF has at least one program supporting simulation.