Some more:

Quick'n'dirty:

    .model small
    .code
     org 100h
    start:
     int 19h                 ; Bootstrap loader
    end start

More "correct":

    .model small
    .code
     org 100h
    start:
     db 0EAh                 ; Jump to Power On Self Test - Cold Boot
     dw 0,0FFFFh
    end start

Even more "correct":

    .model small
    .code
     org 100h
    start:
     mov ah,0Dh
     int 21h                 ; DOS Services  ah=function 0Dh
                             ;  flush disk buffers to disk
     sti                     ; Enable interrupts
     hlt                     ; Halt processor
     mov al,0FEh
     out 64h,al              ; port 64h, kybd cntrlr functn
                             ;  al = 0FEh, pulse CPU reset
    end start

Great example, a two bytes reboot utility. From the times when we could turn off the computer with a push of a button without fearing a global catastrophe...