TheoraPlay
A small C library to make Ogg Theora decoding easier.
A tiny example to pull data out of an .ogv file is about 50 lines of C code, and a complete SDL-based media player is about 300 lines of code.
TheoraPlay is optimized for multicore CPUs, and is designed to be programmer-friendly. You will need libogg, libvorbis, and libtheora, of course, but then you just drop a .c file and two headers into your project and you're ready to hook up video decoding, without worrying about Ogg pages, Vorbis blocks, or Theora decoder state.