This is a list of the best open-source applications that run on Windows. On this list you will find free, high-quality replacements for the software that you have purchased/pirated. What is open source software? Open source software could be freely shared among users, it came with source code, and it could be improved and modified. On the other side, we have proprietary software, in the sense that users were not allowed to redistribute it, that source code was not available, and that users could not modify the programs.