What's the largest amount of great code you have ever seen work?

I suppose it'd depend on the definitions of "great" and "work". Original moon shots? Latex? In general, once you reach a critical mass of code, it stops being "great" and starts being "mundane", because "mundane" works.

The Erlang BEAM VM is fantastic work of art, from what I've heard. I've tried reading the source, but it's all low-level optimized C code, which goes way over my head.

Linux kernel. Huge, but well organized and relatively easy to read.