[gavaletz]

The Linux kernel. If you look at it really closely it isn't so pretty, but its massive size and overall complexity for me (as an undergraduate) was like looking into the grand canyon. What's more impressive is the speed at which things change and that despite breaking all of the traditional rules for "good" software design practices...it works amazingly well.

[ignifero]

I haven't seen the whole of it, but it's not ugly and doesn't break all the rules of good design.