Free MCS-51 Simulators:

*The 8051 Machine - Windows - http://www.jroweb.de/8051/
 -circa. 2002
*JSIM-51 - Windows - http://web.archive.org/web/20160413014254/http://home.arcor.de:80/jensaltmann/jsdl/jsim-e.htm
 -circa. 2000
*ucsim - DOS and Linux (no TUI) - used by sdcc - http://mazsola.iit.uni-miskolc.hu/~drdani/embedded/s51/
 -circa. 2017
*emu8051: http://www.hugovil.com/projet.php?proj=emu8051&lang=en_US
 -circa. 2014
*Tiny8051 (Windows only): https://sourceforge.net/projects/tiny8051/
 -circa. 2016
*gsim51 (command-line): http://gsim51.sourceforge.net
 -circa. 2013
*Emu51: http://emu51.sourceforge.net
 -circa. 2004
*SiMiKON - GUI using qt - http://web.archive.org/web/20050127071817/http://simikon.sourceforge.net:80/
 -circa. 2000
*Yasmin - GUI using qt - simulates at assembly code level - http://web.archive.org/web/20050213004340/http://yasmin.sourceforge.net:80/
 -circa. 2000
*MCU 8051 IDE - GUI - Lin & Win - http://mcu8051ide.sourceforge.net
 -circa. 2012