Isosceles Stance AKA the 70's cop show stance
Apparently named because the legs form an isosceles triangle shape. A wide stance which I guess has decent stability, although this isn't very good for mobility or forward/back balance. Overall, it's kinda awkward looking.
Named after the same guy who invented the Weaver grip. The legs are shoulder width apart and one foot is slightly behind the other. The standard two handed pistol stance. Cops use the same foot positioning for the "interview stance" in the case that they have to draw their gun or other equipment such as pepper spray or taser.
Modified Weaver Stance
The back leg is further behind the front leg and the shoulders are turned more. This stance is like getting ready to run forward or a fighting stance. Personally, if I was using a handgun, I'd be comfortable with this one.
One Handed Forward Stance
This one is a bit unusual, but I saw this in an instructional video. The non-shooting hand is on the shoulder to keep you facing squared off in that direction and for stability. Also note the slight lean with the gun's sights to accommodate.
One Handed Sideways Stance
The classic badass action movie pose. A decent stance that presents you as a smaller target. Although the problem with this is that the weak point of body armor happens to be the sides of your torso, which you are clearly showing towards your target.
Long Gun Stances
Same exact foot positioning as the weaver stance and the modified weaver stance.