By far the best c compiler i have found for the 8051/2 is a freeware one called sdcc(small device c compiler).Available from sdcc.sourceforge.net
It also can be targeted at the z80,gameboy z80 dallas'390 and to some extent the avr and 14 bit pic devices.The libraries for the various devices are in differing states and i think that development for the dos/windows version has now stopped,but apart from that its very good.
It also has a 805x simulator
Jez Smith