I'm using RIDE51 from Raisonance. It doesn't have an ADC in it, but I simulate routines from programs that use ADCs... you just need a little imagination to change values in ports, registers and see if your code behaves as expected.

RIDE51 isn't "free" but it's evaluation version permits that you use up to 4K of code. My largest program until now doesn't use use 2k...