Excellent online 6502 emulator, simulator with graphics output and sample files

Online 6502 emulator in Javascript with source

Good tutorial on emulating a simple assembly language

Brilliant series of tutorials on Python programming including graphics, games, emulators, assemblers

Good guide on writing a game using 6502 and forth - interesting.

Great simple 6502 emulator in C

Good portable z80 emulation library written in C

Good documentation on 6502 CPU

z80 simulator for duiniomite with excellent source of z80 simulator/emulator

Supplier of FPGA development tools including Papilio arcade that allows simulation of retro arcade hardware - excellent

Very good 6502 monitor, simulator and disassembler in Python.

6502 emulator written in Python

6502 emulator/simulator in C

Space invaders emulator with source code - good tutorial

Pacman emulator with source - good tutorial

6502 Apple 2 emulator in Python with clock cycle timing

Mame and console emulation package for xbox - awesome!

Excellent dos and pc emulator for variety of platforms

Excellent emulator and online version of classic BBC micro games such as Repton and Chuckie Egg

Excellent ZX spectrum online emulator

Open source hand held games console that can emulate a wide range of systems such as Amiga 500 and N64

Elite Gaming The arcade machine specialists and home of the Mega Arcade range of arcade machines. Excellent range of multi arcade machines that are based on multi game jamma boards rather than emulators