The main support weapons are bombs, because the main type of support needed is during heavy fights with huge numbers of enemies, where single target damage begins to lose its appeal. Bombs with interesting effects almost always end up being support oriented as well, which further adds to their appeal as support weapons.
But what about support weapons that what not are bombs? I've had the most luck with volcanic pepperbox and sudaruska. Both of these allow me to charge up attacks, and hit enemies that are assaulting my team members, freeing them of a variety of mobs. Blitz needle and a number of other weapons allow me to pick off mobs attacking team mates as well.
How do you use support weapons, and what would you like to see?
Shocking salt bomb makes for an interesting support weapon, in addition to shock status it also has a great flinch rate and is capable of getting behind mortafires easier than most other weapons.
The Dark Briar Barrage DBB can effectively shut down beast and gremlins making it easy for your party to get in a Flourish or Acheron charge.
The Obsidian edge is capable of dealing medium range strong poison, very useful when assaulted by a large horde of monsters that are supported by healers.
Divine avenger makes for a great support weapon, while it's DPS isn't better than brandishes it has the power to move huge masses of monsters with the charge.
The FOV is what I consider a support weapon since it has the power to destroy some of the worst monsters in SK (Trojans, lumbers, and Deadnaughts) making fights much easier.
The Magma driver is a pretty good weapon, it really fits the gun and run method and is very useful for dealing good DPS in mixed monster crowds.