Batch, 10 bytes

On windows, this would suffice

shutdown/s

This will set up a shutdown in one minute.

Alternatively, as @flashbang points out:

shutdown/p

This shuts down the computer immediately.

GRUB shell, 4 bytes

Golfed

halt

Command: halt --no-apm

The command halts the computer. If the --no-apm option is specified, no APM BIOS call is performed. Otherwise, the computer is shut down using APM.

PowerShell, 13 bytes

Stop-Computer

Pretty self-explanatory? Doesn't work on PowerShell core, so it won't work on PowerShell for Linux/MacOS (already tried it on TIO :-D ).

Bonus Garbage Submission, 12 bytes

gcm s*r|iex

In my testing, this works on Windows 2003 with PowerShell 2.0. It won't work on most of the newer versions, and it may not work depending on which modules are installed. It's quite terrible!

This searches the list of commands for any with the pattern s*r and then runs all of them! To be clear, this is dangerous, don't do this!

On my 2003 machine, the 3 commands it returns are:

CommandType     Name                                                Definition                                         
-----------     ----                                                ----------                                         
Application     scrnsave.scr                                        C:\WINDOWS\system32\scrnsave.scr                   
Application     ssmarque.scr                                        C:\WINDOWS\system32\ssmarque.scr                   
Cmdlet          Stop-Computer                                       Stop-Computer [[-ComputerName] <String[]>] [[-Cr...

Yes, it does launch the marquee screensaver, but no, it does not wait for it to complete before launching Stop-Computer.

This approach can work on newer machines, again depending on which modules are installed, but the best I could do on Windows 2012 and 2016 was 14 bytes:

gcm sto*er|iex

(and yeah, sure, the winbatch submission will work in PowerShell and it's shorter, but.. that's no fun)

MATLAB, 11 bytes

!shutdown/p

Somewhat similar to Stewie Griffin's approach. However, MATLAB has way shorter ways of invoking system commands; in this case, ! is used. Windows does not need .exe for command names, so that's left out as well. The / option is still supported (I'm running Windows 10), and negates the need for the space. Alternatives are:

system shutdown/p % 17 bytes; uses so-called 'command' syntax.
dos shutdown/p    % 14 bytes; the 'dos' command can be used just as well.

Matlab, 21 Bytes

system('shutdown -s')

I haven't tried this (closing all programs to do a restart in the middle of the day isn't really tempting), but it should work.. This works...

I'll add a short explanation later, my laptop is currently off.

Bash, 7 6 bytes

init 0

Assuming that the program is run as root.

Assembly (x64, Linux, gcc), 22 21 bytes

xor  %eax,        %eax
mov  $0x58,       %al
mov  $0xfee1dead, %ebx
mov  $0x28121969, %ecx
mov  $0x4321fedc, %edx
int  $0x80

Must be run as root. This is equivalent to pressing the power button.

To prevent file corruption, make sure you execute sync first.

Byte count

objdump shows the following generated byte code.

4004fd:       31 c0                   xor    %eax,%eax
4004ff:       b0 58                   mov    $0x58,%al
400501:       bb ad de e1 fe          mov    $0xfee1dead,%ebx
400506:       b9 69 19 12 28          mov    $0x28121969,%ecx
40050b:       ba dc fe 21 43          mov    $0x4321fedc,%edx
400510:       cd 80                   int    $0x80

Commodore 64, 11 bytes

0 sys64738

This is from the BASIC prompt. Or...

Commodore 64, 6502 assembly, 3 bytes (assembled), 16 bytes (source)

*=8000
JMP $FCE2

SmileBASIC (3.3.2), 14 bytes

READ VAR(A)[0]

Triggers a crash causing the 3DS to restart. I hope this counts.

Java (versions 8+), 101 98 bytes

Windows

interface C{static void main(String[]a)throws Exception{Runtime.getRuntime().exec("shutdown/s");}}

Python 2, 33 bytes

import os;os.system('shutdown/s')

This just might be the least interesting or helpful code I have ever written.

C++, 51 bytes

#include<os>
void Service::start(){OS::shutdown();}

Only works in IncludeOS (v0.10.0-rc.1)

Test:

$ pwd
/home/simon/IncludeOS/examples/demo_service

$ cat service.cpp
#include<os>
void Service::start(){OS::shutdown();}

$ mkdir build && cd build && cmake .. && make && cd ..
[...]

$ boot build/IncludeOS_example
[...]
================================================================================
================================================================================
 IncludeOS v0.10.0-rc.1-dirty
 +--> Running [ IncludeOS minimal example ]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

$

Groovy, 23 bytes

'shutdown /s'.execute()

AutoIt, 11 bytes

Shutdown(1)

Shutdown function gets combination of following codes

0 = Logoff
1 = Shutdown
2 = Reboot
4 = Force
8 = Power down
16= Force if hung
32= Standby
64= Hibernate

To shutdown and power down, for example, the code would be 9 (shutdown + power down = 1 + 8 = 9).

Clojure, 40 39 bytes

(.exec(Runtime/getRuntime)"shutdown/s")

-1 byte by changing the flag from -s to /s, which allowed me to get rid of the space.

Basically the Java answer.

Testing this was fun. Windows decided to do a firmware update on my Surface the second it shut down. Of course.

(defn shutdown []
  (.exec (Runtime/getRuntime) "shutdown -s"))

Spectrum Basic, 3 bytes

NEW 

(not sure how these are counted - 1 token)

C++, 48 47 (Windows) 46 45 (Linux) bytes

Removed 1 byte thanks to Snowman

#include<cstdlib>
main(){system("shutdown/p");}

This should shutdown the system for windows

#include<cstdlib>
main(){system("poweroff");}

This should shutdown the system for Linux

C# 79 bytes

This works on Windows:

class _{static void Main(){System.Diagnostics.Process.Start("shutdown","/s");}}

Node.js, 39 bytes

Linux

require('child_process').exec('init 0')

J, 14 bytes

2!:0'poweroff'

Works on Linux systems. 2!:0 executes the string in /bin/sh.

AppleScript, 45 bytes

tell application "Finder"
 shut down
end tell

I don't know how AppleScript syntax works or anything, so if anybody knows how to shorten this, please tell.

Also, I'm doing this on my phone at school ("hacking"), so I can't test it.

AHK, 19 15 bytes

Run,Shutdown -s

Works on windows only.

Mathematica on Windows 16 Bytes

Invoke the shell command with Run.

Run@"Shutdown/p"

Bash 15 bytes

shutdown -h now

alternatively : init 0

R, 20 bytes

system("shutdown/s")

AutoHotkey, 11 Bytes

Shutdown, 9

AHK Supports a Shutdown command by default. The 9 is a flag, consisting of Shutdown=1 + PowerDown=8.

AWK, 39 bytes

#!/bin/awk -f
BEGIN{"poweroff"|getline}

Assumes running on Linux and that AWK is located at /bin/awk and that the poweroff command is in the users path.

Since it uses the BEGIN block, no input is necessary. It simply pipes the output of the poweroff command into getline which it precedes to ignore.

Should work for Windows users provided they are in an environment similar to Cygwin.

JavaScript (non-competing), 9 7 bytes

Because of JavaScript's nature, closing the current tab is as close as you get (and as such, you can never really complete the challenge without abusing some bug in an OS). I still figured it's nice to have an answer in every language, so here goes:

close()

AppleScript, 30 bytes

tell app "Finder" to shut down

Does exactly what it says: tells Finder to shut the computer off.

Batch, 8 bytes

shutdown

Hibernate ya' want?

shutdown -h

What about a restart?

shutdown -r