Sourceforge is an excellent repository of free software for Windows, Linux, MacOS and even Android. All the software is open source, meaning that the source code is also made available for other programmers to edit. This provides for a collaborative approach to software development.

Sourceforge hosts a number of well-known application, including VLC media player, Audacity sound editor, and Pidgin IM. It also provides Games, Educational, and Science & Engineering software. So go and browse the directory of software to see whether there is anything that you may like.