Notes and a DOS-based PDP-8 CPU simulator (works well in DOSbox/Dappelgrey) from the author's comp org course at Wittenburg. This is a great sim for beginning PDP-8 programming, although it has no extensibility. For its purposes tho, it's excellent.

http://www4.wittenberg.edu/academics/mathcomp/bjsdir/PDP8HomePage.htm