From the Sherpa team:
On April 20, 2015 Sherpa became an Open Source project! The source code repository is now on GitHub. The project repository can be 'cloned' allowing for development of the new code, new extensions or bug fixes. The Sherpa Project welcomes contributions via GitHub.
Sherpa is a modeling and fitting application for Python. It contains a powerful language for combining simple models into complex expressions that can be fit to the data using a variety of statistics and optimization methods. It is easily extensible to include user models, statistics and optimization methods.
Now, you can build Sherpa from source on your own platform within your Python environment with additional customization. See more details on Sherpa documentation page.