The Machine Language Tutor by the NorthWest group of California - I'm not a ML programmer, but if I wanted to get started, I would definetly give this intro to 6502 program some serious attention.  This program is really cool in how it walks you through all the various machine language instructions, and shows you in simple animations how the bits in the CPU registers and memory locations update as the code executes.  This one is worth looking at, even if your just curious about ML and don't really want to learn to program in it.

This archive includes the full NorthWest Starter Kit.  There are some other utilities / programs you might find useful on it.