Power LED:

Super easy mod, can install any color you like, or a color changing LED, like this one.

snes-purp-led[1]

$5 for single color LED.
$7 for color changing LED.

SFC mod:

Another easy mod, removing the tabs inside the throat of the cart slot will allow a US SNES to play Japanese (Super Famicom) carts.
$10

Lockout mod:
This will allow you to make your US system region free.
A switch is used to re-enable the lockout chip to be sure all US games still function properly.
SFC mod also performed since JP carts have different case.
$25

snes-ff4[1]

S-Video reconnect:

This mod restores s-video output on the original a/v connector on the back of the console. You will need the right a/v cord obviously to get the s-video signal to your tv. Cheap cords are available on ebay for about $5 shipped!
$35

 snes-svid-cord[1]

Repairs:

Fuse:
If your system won’t power on and you’re sure the a/c adapter is good then the internal fuse is most likely blown, easy fix.
$5.

Power Jack:
Another notorious issue is the power connector breaking.
I haven’t found a snes style replacement jack yet, but can easy install a jack that accepts a more common plug (5.5mm x 2.1mm).
$15

SNES2-brokenDCport[1] snes2-powerjack-fix-blacksnapin[1]