ESP is a free, open-source program that can be modified by any experienced programmer. The current version of ESP is 4.0.
To modify ESP, you'll need a computer running Linux. To modify ESP Desktop, you'll need a Linux computer (for common components) and a Windows computer with Visual Studio.NET (for the Windows-specific parts). See the documentation section "Modifying ESP" for the full requirements for programmers.
Remember, you don't have to be a programmer to configure and use ESP and create your own experiments. The source code is needed only for changing the internals of ESP, ESPPrefs, and ESP Desktop.
You can also download a compiled package for ESP, ready to install.