I know a SPAS-12 can be modified to be like this, but are there any others?

Erm, pistol grip shotguns (edit for clarity SEMI AUTOS, non semis are a critter of a different stripe)  fall within the 'Assault Weapon' Ban bullshit.

If they dont have over a 5 round tube they can have a 'pistol grip' as their 'evil feature'.

Here is the 'AWB definition:

D) a semiautomatic shotgun that has at least 2 of-          (i) a folding or telescoping stock;          (ii) a pistol grip that protrudes conspicuously beneath          the action of the weapon;          (iii) a fixed magazine capacity in excess of 5 rounds;          and          (iv) an ability to accept a detachable magazine.

One other thing if it 'accepts a detachable magazine' of 10 rounds (?) or more, it has probably been redefined as a 'destructive device' all NFA rules then apply.

Quoted: I know a SPAS-12 can be modified to be like this, but are there any others?

Do you mean pistol grip *only*, or a pistol grip/shoulder stock?  Any semi should be able to mount a pistol gripped "assault" stock but most semi's have the recoil spring housed in the buttstock so the pistol grip only is a no go.  The Mossberg M5500/9200 has the recoil spring around the mag tube so it is possible to shorten it up with a pistol grip.  Still not a good idea because the Mossberg top mounted safety becomes real awkward & slow to use with a pistol grip.

I mean just a pistol grip, no stock.

Doesn't Benelli have an M3S90 with a folding stock? This would be similar to the SPAS with the hook. I half remember seeing a Remington 11-87P with a pistol-grip-only at a local LE supply shop. This was a few years back. I haven't seen it anywhere else, however.

Most semi-auto shotguns have a bolt spring tube that extends almost the full length of the buttstock. Thats why you usually don't see just a pistol grip on a semi-auto.

Thanks,
Lee
www.thearmsroom.com