A Mark IX is a naquadria enhanced nuclear bomb, also known as a "gate buster".
Developed primarily by Samantha Carter, it is a long cylindrical device meant to destroy everything within a 100 miles radius, including the otherwise indestructible Stargate. It has a several (at least 10) gigaton yield. When deactivated, it is in "mode 3", yet from "mode 2" it is extremely volatile. Any effort to disrupt, alter or destroy the device will lead to the weapon detonating immediately.
It was used in an attempt to prevent the Ori from getting a foothold in the Milky Way galaxy at Kallana. Because of its destructive power, Cameron Mitchell was very nervous about using the bomb. The Ori technology however only absorbed the energy, because it was working with a black hole as a source. The blast gave the shield emerging from the Stargate 70% of the power needed to cover the planet. (SG1: "Beachhead")
When SG-1 needed to deactivate the Ori Supergate in order to send the Sangraal to the Alteran Home Galaxy, the Daedalus was sent to detonate a Mark IX behind the Stargate over the black hole. This was a successful attempt, demonstrating that the Mark IX is capable of destroying a Stargate from its more vulnerable back side. Carter, after checking the information given to them by Daniel, stated that this would work as the wormhole wouldn't be able to absorb the force of the blast as it would if the Mark IX was detonated from the front. (SG1: "The Shroud")
When the Asurans took control of Atlantis, Dr. Bill Lee suggested using a Mark IX to destroy the Midway space station to prevent the Asurans from reaching the Milky Way galaxy by using the McKay/Carter Intergalactic Gate Bridge. (SGA: "The Return, Part 2")
In retaliation of a possible Asuran attack on Earth, Atlantis sent the Apollo to use the Horizon Weapons Platform to destroy the Asuran ships. Horizon was designed to carry six Mark IX and four decoys in order to launch a counterattack in order to destroy these ships from planetary orbit. (SGA: "First Strike")
While searching for the Ark of Truth, Col. Carter suggested placing four Mark IX between the parts of the Supergate to destroy it when the gate is dialled again the next time. (SG1: "The Ark of Truth")