I'll throw in another vote for the M1 carbine (bonus points for a folding-stocked M1A1). Sure, it doesn't have the range or energy of a battle rifle, but for short range or civilian defensive use, I think it's a better PDW than many of the modern designs. The "Enforcer" pistol variants are close to silly, but a 30-round magazine of .30 carbine in a package that small has some interesting possibilities.
I'm also a big fan of technological dead ends and might-have-beens. Accordingly, I'll present something that saw only very limited service with U.S. forces: the Johnson M1941. Image is linked to the Box o' Truth article:
.30-06 semi-auto, internal rotary magazine, and just looks cool as the day is long. Definitely on the list for my C&R collection when I win the lottery.
- C.