Arguably THE open source software that revolutionized screen recording and live streaming. Not just software.
XSplit emerged slightly before OBS Classic but OBS innovated with a far superior game capture implementation on Windows, supporting shared texture hooking across D3D8-D3D11 and OpenGL, which helped it gain massive traction with the Twitch crowd. After retaining a large user base it was eventually rewritten as the cross-platform OBS Studio.