Plasma beam weapons are a highly-effective ship-based weapon system developed by the Asgard just prior to their extinction in 2007 and gifted to the Tau'ri as part of their legacy. They are among the most advanced ship-based weapons known to exist.
Unlike most ship-based energy weapon systems, which are designed to fire individual pulses of energy, the Asgard plasma beam is a narrow, high-intensity beam of superheated plasma. These beams are capable of overloading and penetrating even the most advanced shields and burning through the hulls and internal structures of an enemy vessel with comparative ease, causing heavy damage; if the beam hits a critical area the target can be destroyed in only two to three shots. This incredible ship-to-ship weapon might be one of the most powerful and efficient ever encountered. (SG1: "Unending") (SGA: "Be All My Sins Remember'd")
In terms of effectiveness, Asgard plasma beam weapons are among the only known weapons capable of penetrating the shields of Ori warships, being able to destroy such vessels in 5 to 6 hits (the shields begin to fluctuate after 2 hits, indicating that they are being drained, and fail after 3-4 hits, with the ship itself then sustaining crippling damage). (SG1: "Unending") For reference, pulse weapon fire from an entire fleet of ships (including ten Ha'taks of the Free Jaffa Aliance, three Ha'taks of the Lucian Alliance, two Tauri BC-304s, and an Asgard O'Neill-class ship) were unable to penetrate Ori warship shields. These weapons have also proven exceedingly effective at destroying unshielded Asuran Aurora-class battleships in two hits, and Wraith Hive ships in three hits. (SGA: "Be All My Sins Remember'd", "The Last Man") The power of these weapons furthermore appears to be adjustable; Two shots from a BC-304 were used to incapacitate a Wraith cruiser without destroying it. (SGA, "Search and Rescue")
During the Battle of M2S-445, the Daedalus was able to use its beam weapons to disable the hyperdrive of Michael's cruiser with two shots to the stern as it attempted to flee to hyperspace. The Daedalus later destroyed Michael's cruiser with two more shots from its beam weapons after Teyla Emmagan was rescued and the cruiser was disabled. They were later used in the Battle of the Super-hive, but were largely ineffective against the upgraded hive ship due to its much-improved hull armor, with a direct hit from the Daedalus resulting only in minimal damage. The Apollo and Sun Tzu presumably used these weapons in their later attempt to destroy the hive ship to no effect as well despite there being two ships at once using the weapons. (SGA: "Search and Rescue", "Enemy at the Gate")
During the Battle of M35-117 in an alternate reality, an alternate Daedalus attacked an Alien ship with these weapons. While the few shots it was able to get off due to great lack of power weren't enough to destroy the alien ship, it disabled the ship's weapons and saved the alternate Atlantis. (SGA: "The Daedalus Variations")
In an early battle with the forces of Queen Death, the George Hammond lost use of its plasma beam weapons and they were determined to be beyond repair until the ship could return to Earth. As a result, the Hammond lacked its plasma beam weapons during the following battles with Wraith forces. (SGA: "The Lost")