6502.org

Hi.

I've found this 6502 emulator with immediate assembler and Atari 2600 support and it works really well.

http://8bitworkshop.com/

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.)

New platform in 2016 for 6502, now that is an interest experience!