I've played demo extensively. Is probably my second favorite perk (especially once I got it over 3 or so. Too expensive before that). I generally use m32/m79 with pipes sometimes (only if I have tons of cash and people are funneling me money.)
On maps with nice hold spots, I can use the m79 on ceilings and high walls to either wipe out or stun whole groups without much smoke (high smoke doesn't seem to block sight as much as low flying for some reason) Elsewise the M32 will be able to take out Fleshies or nasty sirens as needed. Pretty simple, shouldn't need much explaining.
My first thought as alternate loadout would be M32/LAR. Lets you take out crawlers easily and even stalkers in one shot on lower difficulties. You're giving up your ability to both hold a hallway down AND kill big enemies (assuming you are needed for both jobs) and become more of a support role. Though I'd like to note for crawlers coming at you, it's not hard to pop off 3-4 9mm rounds and simply kill them. For clots/gores/etc coming at you on Sui/HoE, 9mm won't cut it, which is probably why you have the LAR... though the LAR isn't as reliable in this instance since it's single shot reloading and takes awhile to do so. (Also would like to note that HC fit in here and are a better alt weapon than LAR on lower difficulties. Until suicidal, both 1 shot crawlers for example, but the HC can reload more quickly. Disadvantage is you can't switch to 9mm as quickly since you have 2 weapons on the 2 weapon slot).
This leads me into the next loadout I like, M32 and AK/SCAR/BP. The Bullpup is a bit of a stretch, but on lower difficulties, it pops heads easily enough even without the perk damage increase and recoil lessening. On higher difficulties, an AK on single fire can take off heads and likely drop the middle range stuff. You won't be able to fire too quickly due to recoil, but full auto can easily take out a group of crawlers too close. Maybe I'm jaded here because Commando IS my favorite class and I'm extremely comfortable with all of them, perked or not. Today I used the M32/Scar/Pipes combo on HoE Mountain Pass. It worked extremely well too. Single shot scar can pop heads at medium long range acceptably well and we had an xbow in the group to stop husks from sniping us. I was able to kill gores, stalkers, clots, and crawlers effectively one at a time at range. Even managed to kite a bit and survive a team wipe. Of course that was short lived because we managed to pull it back together to the defense point. Personally my belief is this, on higher difficulties the enemies come too fast and too hard to really USE a second unperked weapon. As far as they go, the AK/Scar both allow for short bursts of head popping (no pun intended on burst fire) with single shotting, yet conserves ammo for your main gun. Though in a pinch I'd rather waste a grenade and point blank shoot the crawler biting me than to switch to ak and take another hit or two trying to kill it.
I saw mention of medic gun elsewhere. This plays into another combo I like. Katana + M32 + Mp7m. This is primarily a medic combo to me, but can be applied to demo somewhat effectively. This was back before scrakes got redone, so you could katana back pedal and slowly take off a scrakes head (like zerkers now do with FPs) and the Mp7 allows for crawler killage and team heal-age. Good support stuff. Then your bread and butter killing is done with the M32. Since you have a katana you can do short kiting runs as a demo (but more as a medic) thanks to the melee speed boost. You won't feel as powerful as you do with other weapon loadouts, but you'll be more useful in support of your team. As I noted before, it is more of a medic combo to me, but either way thrown nades + m32 drum can take out a FP even unperked at middle difficulties.
I can't really think of too many others I like or have used too much. My friend likes shotguns, so M32 + pump shotty would make sense. He seems to feel that its easier to kill crawlers with a pump shotty than anything else. So that's an option. But I don't really care for it. I guess M32 + Mac10, but inaccuracy rather ruins the gun for that loadout. Unless you want a few free points in fire damage I would avoid that one.
That's all I can come up with as far as demo weapon combos.