The best one if you can afford it is keil, as far as I remember you cant use keil simulator for eval version.
Keil combinde with PClint is a professionals dream.
Read the getting started manual that comes with the eval version of keil, it has a wealth of information that makes you need very little reading anywhere else.
You can try Raisonance also, their C compiler runs up to 4k, also SDCC is a sharewhere c compiler with some bugs in it.
I don't know assembly of 8052 so I can't advice you of assemblers.
Regards
Mahmood