The Open System for Earthquake Engineering Simulation (OpenSees) is a project of the Pacific Earthquake Engineering Research Center. OpenSees is an open source "software framework for developing applications to simulate the performance of structural and geotechnical systems subjected to earthquakes." The project's homepage maintains resources for users and developers, including downloadable source code, extensive documentation, and instructions on how to contribute code. Several links to other research projects that are using the OpenSees tool are provided. Additionally, an OpenSees user's workshop was held in August 2003, and the presentations from the event are available.