Plasma Beam

The Plasma Beam is a beam which appears in all of the Metroid games except for the original Metroid, Metroid Prime Hunters, and Metroid Prime 2: Echoes (though she gains the Magmaul in Hunters and the Light Beam in Echoes, which have similar properties). In 2D incarnations, it is a thin ray capable of piercing multiple enemies; in the Prime Series, it appears as a short-range beam of intense heat that can be used to melt ice and weld metals. The Plasma Beam also appears in Metroid Other M, right before the Ridley fight. Samus's Gunship is also stated to be equipped with a "retractable rear-mounted plasma cannon".

In the Metroid Prime series
The Plasma Beam was the second to last beam acquired in Metroid Prime, and as such, apart from the Phazon Beam it is perhaps the most powerful beam weapon in the game. When charged, it could instantly incinerate some normally resilient enemies such as Baby Sheegoths and Plated Beetles, and it could set fire to others, causing damage for a few moments after the shot hit them. It was also capable of melting Frigidite casings. The Plasma Beam's color scheme in Prime was red, and it represented the "element" of fire. It can open Red Doors as well. The Flamethrower, the Charge Combo for the Plasma Beam, was a continuous Charge Combo that shot out a stream of flame with an exceedingly short range. It was primarily useful for killing Plasma Troopers and cold-based enemies such as Sheegoth.

The Magmaul in Metroid Prime Hunters takes over some of the Plasma Beam's functions. In Metroid Prime 2: Echoes, the Light Beam was very similar to the Plasma Beam with its heat-based and burning effect as well as with its animation. However, it was the Annihilator Beam that used the Plasma Beam's Arm Cannon shape from Metroid Prime.

In Metroid Prime 3: Corruption, the Plasma Beam is acquired after defeating the hunter Ghor. The beam was no longer short-ranged, and the rate of fire remained the same as the Power Beam as all beam weapons were stacked upgrades in Corruption. To compensate for this heightened firing rate, however, the Plasma Beam suffered a downgrade in firepower when compared to Metroid Prime. Its Arm Cannon shape was altered as well. No longer is the gun extended forward -- instead, it takes after the Ice Beam from Metroid Prime and the Dark Beam from Metroid Prime 2: Echoes by being slightly elevated upwards. It retains its ability to ignite enemies and melt objects seen in previous games, and can open Orange Doors. In addition, the newer Plasma Beam in Corruption is capable of repairing damaged cables and circuitry with a beam-focusing attachment; this ability is kept even when stacked to the Nova Beam, though the mentioned attachment gains a green glow. http://www.youtube.com/watch?v=ls0j5R98cFI

In the 2-D games
In Metroid: Zero Mission, the Plasma Beam was gained as an unknown item and could not be used until the Power Suit was regained in Chozodia. The Plasma has the unique capability to pierce through multiple foes, ultimately obliterating them. When combined with the Wave Beam, it becomes virtually unstoppable and extremely destructive as each shot will pass through both hostile enemies and solid material.

In Super Metroid, the Plasma Beam was incompatible with the Spazer Beam. If both were simultaneously selected in combination with a variety of other factors, the game would glitch, creating results such as the Murder Beam, Spacetime Beam, and the Chainsaw Beam.

In Metroid Fusion, the Nettori possessed this function, and Samus was able to recover it when she absorbed the X Parasite it left behind. According to Adam, the Plasma Beam was the only thing that could pierce the SA-X's armor and damage it. This implies that the Varia Suit is highly vulnerable to the Plasma Beam's piercing effect.

In Super Smash Bros. Brawl, Zero Suit Samus has two attacks called the Plasma Whip and "Plasma Wire" that fire from her Paralyzer. Both also act as a tether, similar to the Grapple Beam she can use with her armored Power Suit.

In Metroid: Other M
The Plasma Beam returns yet again retaining its piercing abilities from the 2-D series, and of course, the last beam to be obtained being the strongest beam of all. Adam mentions to use its properties against an Adolescent Ridley and ultimately destroy it. When it evolved into the adult Ridley and confronted Samus in the Geothermal Power Plant of the Pyrosphere, Adam authorized the Plasma Beam so Samus could ultimately destroy Ridley.

Anthony Higgs has a type of Plasma Cannon, though he complained about it of how it takes "forever to charge". He used it twice against Ridley in his premature and adult forms. It also has a infra-red laser attached, shown when he aimed it at Samus which was intentionally aimed for Ridley.

Metroid II: Return of Samus manual
"This extremely powerful weapon fires a beam that can even penetrate the solid walls. It is the strongest and most destructive of all of the beam type items."

Super Metroid manual
"The most powerful of all the beams can destroy several targets at once."

Official Metroid Prime website
Weapon effect: Expels bolts of pure molten energy.

Weapon range: 3 - 10 m

Weapon potential: Lethal

"Chozo Technology. The Plasma Beam is expelled from Samus Aran's Arm Cannon. Capable of firing bolts of pure molten energy, this weapon becomes even more effective when charged. Extremely effective against ice-based targets. It is perhaps the most potent weapon in Samus Aran's arsenal."

Metroid Prime manual
"The most powerful beam weapon in Samus's arsenal, the short-range Plasma Beam launches streams of molten energy that can ignite some enemies and melt ice. Like all beam weapons, the Plasma Beam can combine with Missiles for a high-powered combo attack once Samus finds its combo power-up."

Metroid Fusion manual
"This beam passes through enemies, hitting multiple targets."

Zero Mission Samus Screen data
"This powerful beam can blast through multiple enemies."

Metroid.com
"Untold levels of destruction come from a shot of Plasma, which can even pass through enemies themselves. Precious few foes are immune to the power of this beam."

Metroid: Other M manual
"Adds the ability to pass through enemies to Samus's beam. This most powerful beam can also be shot as a Charge Beam."

Trivia

 * Interestingly, the Plasma Beam in Metroid Prime is the only beam that increases in range when charged (albeit only slightly).
 * Strangely, in the New Play Control! and Metroid Prime Trilogy versions of the game, embers no longer float off the Arm Cannon when charging. This may be due to the way this and other visual effects were programmed in the original versions of the game, making it too difficult to reproduce the effects on the Wii. It is also possible this had to be removed due to the free-aim features of the Wii controls.
 * In Corruption, when Samus tests the Plasma Beam out during the pickup cinematic, the sound it makes is the same as in Prime, although it makes a different sound in Corruption.
 * The Plasma Beam, along with Samus and several of her other abilities were slated to appear in Marvel: Ultimate Alliance, but they were removed due to Nintendo withholding their consent. The Plasma Beam in Ultimate Alliance appears to be based off of Prime's iteriation due to its orange color.
 * In the first Metroid Prime, the symbol for the Plasma Beam is a hand print presumably combining the index finger with the middle finger, the ring finger with the pinkey, and putting them together, with the thumb turned inward toward the hand. Using the X-Ray Visor reveals that Samus configures her hand this way to activate the beam.
 * Near the end of Metroid Fusion, Adam stated that the Federation completed downloadable data for the Plasma Beam, but withheld it to stop her from destroying the SA-X, and thus preventing their studies. However, Samus restored the function on her own.
 * The Plasma Beam in Metroid: Other M looks very similar to the Nova Beam in Metroid Prime 3: Corruption.