Easy6502 by Nick Morgan is one-stop accessible tutorial on 6502 assembly language programming, including a series of worked example programs which you can edit and run in the embedded emulator.
See http://skilldrick.github.io/easy6502/ for the live site.
This (original) fork is now in a strict maintenance-only mode. Pull requests are welcome for bug fixes.
Please see other active forks for further refinements and developments of the tutorial and the emulator: https://github.com/skilldrick/easy6502/network