A good place to start is this interactive code viewer: http://www.keyframesandcode.com/resources/javascript/deconst... There is also a great presentation by Paul Irish ("10 Things I Learned from the jQuery Source"): http://www.youtube.com/watch?v=i_qE1iAmjFg
Take a look at anything in OpenBSD (ex: http://www.openbsd.org/cgi-bin/cvsweb/src/usr.sbin/bgpd/bgpd...) or nginx (http://nginx.org/download/nginx-1.0.0.tar.gz).
I had to go look. I agree; there are some things done consistently in the code that are high on my nitpick "don't do that" list. Ugh. I'd have to reformat to work on it, then format to whatever standard they use on checkin, I think.
On the other hand, if you look at the question in a different way, you could say that the code (be it open source or not) every programmer must read before his end, is his own. As one looks at his life in one's deathbed, so should a programmer look at his code.
Quite the contrast with the Bourne Shell: "Nobody really knows what the Bourne shell's grammar is. Even examination of the source code is little help." – Tom Duff http://en.wikipedia.org/wiki/Bourne_shell#Quotes
[edit] michaelcampbell has already recommended it below - missed that.
It is so easy to understand inner workings of window manager in X reading this code, and you do not need much time for it (>2000 SLOC).
Passenger - code looks good even though it's C++
Rails 3. Beautifully structured application foundation that's not just a pretty piece of code, it's tested and works.
JScheme. Reading this clean implementation and re-implementing it yourself gives a good basis of understanding for lisp.
twisted is, well, twisted. And that's what it does to your head too.
If you just want to die by reading code, you can try to understand how Xen works ;) (disclaimer, it is actually very elegant)
1. The Racket source code
2. The Chromium source
http://en.wikipedia.org/wiki/Lions_Commentary_on_UNIX_6th_Ed...
Get the source code from http://minnie.tuhs.org/cgi-bin/utree.pl