Code which every programmer must read before dying

Probably the most funny source code I encountered was that of the linux kernel, just look for comments.

If you just want to die by reading code, you can try to understand how Xen works ;) (disclaimer, it is actually very elegant)

WordPress - because it's extremely crappy but at the same time insanely accessible. Lots of lessons to learn.

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.

If you have only few minutes before execution, and want to fix an old X programming mystery, I would suggest dwm.c: http://hg.suckless.org/dwm/file/e901e70f69e8/dwm.c

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).

Plan 9 source code (http://plan9.bell-labs.com/plan9/), mostly for its historical value - created by the same team that made C and Unix - and for an insight on how those people envisioned the evolution of Unix.