[ucs]

As luck would have it, the very same site under discussion also happens to host a neat online "IDE" for learning all about the 6502-based Atari 2600¹.

It is a companion to the book, "Making Games for the Atari 2600". A wonderfully clear and concise primer that I thoroughly enjoyed, even though I had no interest in the Atari 2600 per se. The NES was my first console -- and naturally the one I'd always wanted to program. Since it shares the same 6502 processor, I used this book as an introductory text before diving into the murkier waters of NES development wikis and forums.

¹ http://8bitworkshop.com