I've mentored with x264 (for Videolan) for 3 years, this year going to be the 4th.
It's an absolutely amazing program and has led to an enormous amount of success for our students so far. Almost all of our successful students have gone on to get real jobs (contract, part-time, or full-time) from their work on x264. Most had absolutely no experience in anything they worked on before they started.
Examples:
Holger Lubitz (2008): Got contract work from Avail Media and CoreCodec for his amazing assembly code skills.
David Conrad (2009): Got hired by Apple due in large part to his multimedia experience with x264 and ffmpeg.
Dylan Yudaken (2009): Used Summer of Code as a reference to get a software dev job in Britain (he's from South Africa).
Oskar Arvidsson (2010): Now working part-time for CoreCodec to improve their assembly code.
Simon Horlicks (2010): Getting paid by Avail Media to finish up his (huge) 2010 Summer of Code project.
Daniel Kang (Google Code-In, 2010): Won top-10 in GCI, got into MIT as a result, and is probably going to get work from CoreCodec. Oh, and he's only 17.
Additionally, x264 has a dual-licensing program -- all successful students, having contributed a large chunk to the codebase, get a share of the profits (like any other major developer).
I did GSoC last year with FFmpeg, working on the RTP/RTSP layer. It was a lot of fun working on a code base like that -- multimedia seemed so arcane before starting out.
One of the things I appreciated the most were the code reviews. I had awesome mentors, and my skills have improved significantly thanks to the feedback I got.
I haven't contributed much since then (although I am working sporadically on NEONifying the swscale library), but I still monitor the mailing lists, hang around in the IRC channel, and collab with one of my mentors every day on a paying multimedia project. Multimedia hackers are some of the smartest people I know, so I hang around them in the hopes of picking up a few things by osmosis.
The only downside to GSoC was the pay, which is quite low, but I got to do things on my own time, like bicycle around and spend half the day at the beach. It was the first really relaxed summer I've had in a long time, coming off 6 years of college -- didn't feel like getting a real job just then. GSoC fit the bill perfectly for me.
I did GSoC under The Mono Project in 2005, and it was a great experience. Especially because Miguel is extremely awesome, on both a technical and personal level.
