Certain Plex components contain open source software, including:
XBMC: The most important one, since the Plex Mac client is based on XBMC code.
Team RazorFish: The talented team who created the lovely MediaStream skin.
FFmpeg: Without this, there would be no decoding of video inside Plex. A critical component, to be sure.
SQLite: The media database is stored using this excellent, industry standard free database.
CxImage: The workhorse code responsible for loading images and photos. It uses lots of other libraries like libJPEG, libPNG, etc.
FreeType: This code is responsible for the beautiful rendering of fonts inside the application.
Python: The media center embeds the Python interpreter to provide powerful scripting functionality.