If I correctly understood you, you were asking about cross-compilers for the 8051 and other MCUs. There is GCC but I don't think there is 8051 GCC, (there is for AVR, ARM and M68K I think).
However a nice RTOS product available for the 8051, the AVR, the PIC, and more is the SALVO RTOS from pumpkininc or from Hi-tech. Check it out.
For the 8051 family there are a lot of compiler in the web and a siple rtos may be the ucos is available with source code and a book (search on Elektroda forum) with a very good explanation how it work.