Hi.
I've found this 6502 emulator with immediate assembler and Atari 2600 support and it works really well.
http://8bitworkshop.com/
8bitworkshop.com 6502/Atari emulator
8bitworkshop.com 6502/Atari emulator
Author, IOCCC+JS1K winner, Princess Quest in Evercade Intellivision, MSX/Atari/CV/Intellivision/SMS game dev, Knight of boot sector games, IntyBASIC/CVBasic creator.
Re: 8bitworkshop.com 6502/Atari emulator
That's a good find!
Just to note, there's something somewhat similar, for a different platform: the Dodo simulator by Peter Noyes:
https://play.dodolabs.io/?code=51447f62
(There are a few web-based 6502 emulations for imaginary machines too, see the Emulators page. And there are many web-based emulators for real machines but not so many which act as a development environment - see this collection.)
Just to note, there's something somewhat similar, for a different platform: the Dodo simulator by Peter Noyes:
https://play.dodolabs.io/?code=51447f62
(There are a few web-based 6502 emulations for imaginary machines too, see the Emulators page. And there are many web-based emulators for real machines but not so many which act as a development environment - see this collection.)
Re: 8bitworkshop.com 6502/Atari emulator
New platform in 2016 for 6502, now that is an interest experience! 
Author, IOCCC+JS1K winner, Princess Quest in Evercade Intellivision, MSX/Atari/CV/Intellivision/SMS game dev, Knight of boot sector games, IntyBASIC/CVBasic creator.