166light  .zip      Hitex' simulator for the 80166.
2sq8051   .zip      Generate two square waves at different frequencies, simultaneously.
51forth   .zip      Nearly completed description of each FORTH word in the 8051
51light   .zip      Keil C51 Compiler Light and Hitex HiSIM51 Light simulator
                    (demo -> the linker is restricted to 512 Bytes of code :-(
8031core  .zip      Basic 8031 circuit with memory and serial I/O in GIF format.
8051exec  .zip      8051 Real Time Executive (MDS Technology) (Shareware).
8051faq   .zip      8051 FAQ.
a51       .zip      PseudoSam 8051 Cross Assembler, V1.4.09.
ad1       .zip      A/D code for the `C552.
as31      .zip      Assembler for 8031/8051 CPU, with C source and a simple monitor.
as48_108  .zip      Assembler for 8048/49/50 [1.08].
as80_108  .zip      Assembler for 8080, 8085 and Z80 microprocessors [1.08].
asem5111  .zip      Cross assembler for the MCS-51/8051 family.
bas051    .zip      Converts IBM BASIC to '51 assembly.
basic-52  .zip      Source files for BASIC-52 interpreter.
basic31   .zip      BASIC-52 for 8031/8051 in external EPROM.
boot8051  .zip      Bootstrap program to read an Intel hex file from a PC serial port,
                    dump it into RAM, then run it.
bootstrp  .zip      Hex file Load-and-Go using the 8051.
clock     .zip      Example of real time clock fm Sytronics for 8051.
d51v26    .zip      Fast 3 pass 8051 disassembler.
dcm44     .zip      8044 Real Time Executive (Intel) (Public Domain).
dcx51     .zip      8051 Real Time Executive (Intel) (Public Domain).
debug51   .zip      80C51 code debugging tool from Axxon.
dis_8048  .zip      8048 Family Disassembler.
dis8051   .zip      8051 Real Time Executive (Intel) (Public Domain).
disasm51  .zip      8051 Disassembler.
dump8051  .zip      Memory dump displays contents of ROM on the screen.
eforth51  .zip      eFORTH environment for the 8051.
emily52   .zip      Demo of a 8051/8052 simulator/emulator which provides a
                    powerful debugging environment, offering many features.
forth51   .zip      8051 Forth System.
forthasm  .zip      8051 Forth Assembler.
i2c8051   .zip      I2C code which lets any 8051 variant communicate using the I2C interface.
i2cbits   .zip      I2C single master code for ANY 8051 type controller.
                    'Bit bangs' I2C on port pins.
intrupts  .zip      Demo of extra external interrupts on `C51.
kernel    .zip      An interrupt-driven multi-processing architecture for the 8051.
key8051   .zip      Hexadecimal keypad routines.
lcd8051   .zip      Liquid Crystal Display driving routines.
math51    .zip      Multi-byte math routines for the 8051.
ml_asm51  .zip      MetaLink's 8051 family macro assembler.
monplus   .zip      A re-written and expanded 8031 monitor based on Ron Stubbers'
                    original one.
nohau     .zip      A simulator of NOHAU CORPORATION's EMUL51-PC in-circuit emulator
                    for the entire 8051 family of microcontrollers.
pci2c     .zip      PC to I2C-bus Test program (Philips Corp).
ps85a12   .zip      Intel 8085 cross assembler.
ps96a12   .zip      Intel 8096 cross assembler.
psuedo    .zip      8051 Assembler (Psuedo Corp.).
sine8051  .zip      Fixed frequency sine wave generator using an R-2R ladder.
tasm283   .zip      8051 Assembler (Speech Technology).
tb51      .zip      TinyBASIC for 8031, w/ source files.
tb51ml23  .zip      MetaLink ASM compatible tiny BASIC.
tutor51   .zip  !   TSR help screens with most of the common 8051 device info.
uasm      .zip      8051 Assembler (Custom Computer Consultants).
ultramon  .zip      First-class 8031 monitor program.
xd8051    .zip      F-PC Forth environment for the 8051.

Much 8051 stuff in this section was copied from :

These are great sites; check them out !