Plasma rifle

Plasma rifles or plasma casters are high tech weapons firing superheated bolts of plasma, powered by microfusion cells and heavy energy cells, which found applications in military and industrial fields. The bolt forms in a special chamber, which is then sent down a superconducting barrel and, in the most common models, stabilized by electromagnetic claws upon exiting it.

Initially, the weapon was a bulky, industrial plasma caster, manufactured by Winchester Arms, however, later developments by the US Army would lead to a more compact design, perfectly suited for urban warfare.

Regardless, all types of plasma rifles have a common feature - when a critical hit is delivered, they can literally melt down enemies. While industrial casters weren't powerful enough to dissolve bones too, later ones had no problem converting the target into a (literally) puddle of goo.

Winchester P94
 The Winchester Model P94 is a bulky, industrial-grade energy caster, firing superheated bolts of plasma down a superconducting barrel ending in electromagnetic claws helping to form the plasma bolt.

In the military version, the company has downsized the weapon and revamped its design, making it a man-portable energy rifle, complete with a stock, pistol grip and carry handle (which can, presumably, also mount optics). 

Interestingly, in an interview for Future Weapons Today, US Army Colonel Moretti slammed the design, likely for being bulky and outdated.

Winchester P94 turbo plasma rifle
 The turbo plasma rifle is a modified Winchester P94 plasma rifle. The plasma bolt chamber has been hotwired to accelerate the bolt formation process, resulting in an increase in the rate of fire 

Urban plasma rifle
 Created by the US Army shortly before the Great War, this variant is a lightweight urban warfare weapon. In contrast with the P94, it does not have a superconducting barrel nor the electromagnetic claws - just the plasma bolt chamber and a discharge coil.

This interesting model found in the hands of the Enclave on the East Coast, around 2277.

For the unique plasma rifle in Fallout 3, see A3-21's plasma rifle.

Multiplas rifle
 Based on the urban version of the plasma rifle, the multiplas rifle is a sort of plasma shotgun ('multiplas' being a portmanteau of 'multiple' and 'plasma'), firing a volley of plasma torroids at the cost of a massive drain on the microfusion cell.

Electric plasma rifle
 The electric plasma rifle appears only in the city of Los and the Secret Vault. It's to be manufactured using technology contained in the Secret Vault. It's equipped by the most important ghouls of the Church of the Lost and some super mutants of Attis army.

Turbo electric plasma rifle
 The turbo electric plasma rifle is a upgraded version of the electric plasma rifle, and is found only in the Secret Vault. It's to be manufactured using technology contained in the Secret Vault. It's equipped by the most important super mutants of Attis army and by the champion of the Gladiator Pit in Los, the Wasteland Stranger.

Appearances
Plasma rifles, as described in detail above, appear in all of the Fallout games.