The least lethal thing I currently carry is my knives. I sincerely wish VA would let me carry cough brass knuckles, that'd be like the perfect weapon for IMHO.
Stungun- Takes too long to pull out,or arm. Most have a wist loop you need to wear,and some sort of rather small/dinky safety switch that you slide to arm it,then an actual firing botton. If you have one that DOESN'T have a wist loop,I hope you have strong hands and fast reflexes so the weapon isn't taking from you.
Pepper Spray/OC Pepper/Mace- Be very careful. I've heard many many stories of simple backwinds,or etc where people have breathed in the fumes from their own weapon. According to Kimber lifeacts website, pepper spray is so slow the BG (bad guy) can usually jump out of the way anyway.
Taser-as aforementioned by others, is ratherl arge to carry in addition to a firearm. If Im going to carry something of that size, its gonna be lethal. Besides the cost, last I looked into tasers,I could buy a glock or SA for that...and aren't they only one shot?