Lascaux, though it has low accuracy due to it's unique spread pattern (in the shape of a caveman painting). it has a very fast fire rate when hip-fired and always spawns with the x2 projectile trait. It spawns on the ground in Frostburn Canyon in the cave area with the spiderants, in a pool of water there. I always grab one on normal mode and TVHM when I'm doing the missions and they last a decent amount of time.
Also the Sandhawk (from Pirate DLC) is similar, having a unique spread pattern (in the shape of a flying bird). It shoots 8 pellets per shot at the cost of 3 ammo per shot, and fires 5 shots in a burst. And because the projectiles split after being shot, it is currently extremely powerful when combined with the Bee shield (like massively overpowered) but still only effective at close range or against large enemies.
Hellfire (legendary SMG drops from Scorch) can also replace the fire plasma caster in some cases. The direct damage isn't as high but the fire DOT stacks, unlike every other elemental weapon, so the fire damage can stack up and add up fast. On Siren with boosts to elemental effect chances you can sometimes just fire like 3-5 shots and move on to the next target and that one will just die from the high stacked burn damage.
Still, I would have to say Hyperion Plasma Casters are better in a bunch of situations too, because of the higher DPS and pin-point accuracy.