Mods:

Power LED:
The Genesis model 1 and 2 already have a red power LED that is easily changed and the model 3 is easy to add to!
If you want I can install another LED anywhere and any color you like, or a color changing LED, like this one.

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

Video mods:
The Genesis systems already have an a/v out port with composite video and audio, all you need is the right cord, find em' cheap on ebay.

If you prefer the yellow, red, and white RCA jacks I can install them for $25.

S-video:
I have done a couple s-vid mods for the Genesis, but i'm more interested in getting the component mod working since it will be much better quality. Also, s-video is still not enough of an improvement versus the cost of the mod to worthwhile.

Region Mods:
Widening the slot will allow you to play Japanese games on your US system.
Not necessary on a Gensis 3!
$15

Coming:

Component video:
Since the Genesis is an RGB capable console, a component output mod is on my list of mods to try.

Overclocking:
I'm also going to add an overclock mod, possibly with two rates to chose from. I've ordered the part and will post results when i get it done.