Reboot.com

Using the following MS-DOS commands, a program called "reboot.com" can be created which allows a user to simulate the Ctrl-Alt-Del restart of DOS by merely running the program (ideally from the DOS prompt. (useful for BATch files or for those typists that still use DOS and can enter reboot faster than give the three finger salute)

-------start of commands----------
debug
n reboot.com
a 100
mov ax,0040
push ax
pop es
es:
mov word ptr [0072], 1234
jmp ffff:0000
rcx
11
w
q

-------end of commands----------

Here's a tiny reboot.com.
I believe this is the smallest useful DOS program.

Commands to create it:

This creates a two byte com file which will reboot the system, bypassing the BIOS POST and jumping directly to the boot loader. It's very useful when you dual boot between Win9x and another OS. Just create an MSDOS mode shortcut to this program and you can rapidly switch to the alternate OS. However, If you use EMM386, It sometimes fails to reboot.

QuarterDeck's QEMM stealth memory manager had a similar feature called QuickBoot.