A JavaScript 6502 assembler and simulator which runs in a web browser, forked from: https://github.com/skilldrick/6502js
Enhancements include a speed control (slider), load/save buttons, an 80x25 character memory-mapped terminal screen, and some basic ROM routines.
A hosted copy is available at http://6502.cdot.systems (temporarily at https://matrix.senecapolytechnic.ca/~chris.tyler/6502/) - Click the "Notes" button and scroll through the text box at the bottom of the display for basic documentation, including links to some sample code. Some sample code to run within the emulator is available at https://github.com/ctyler/6502js-code