Plex Media Center is brought to you by
XBMC: The most important one, since Plex is based on XBMC code. We forked several months ago, and continue to pull bug-fixes and new features, albeit in a controlled fashion.
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.
Many others...