Goddard Space Flight Center just released as open source a tool to perform space trajectory optimization and mission analysis. It appears to be quite cool.
The General Mission Analysis Tool (GMAT) is a space trajectory optimization and mission analysis system developed by NASA and private industry in the spirit of the NASA Vision. GMAT contains new technology and is a testbed for future technology development. To satisfy NASA's mandate and maximize technology transfer, GMAT is an open source software system licensed under the NASA Open Source Agreement.
For more info, see the GMAT website, http://gmat.gsfc.nasa.gov