Dimensional portal of unknown origin, presumably constructed of naquadah, capable of transporting an individual into an alternate reality at one touch. The mirror is controlled with a separate control remote, which allows the user to "dial" to different dimensions.
There But For the Grace of God - Daniel Jackson accidentally crosses into another reality via the quantum mirror, gaining insight into the future fate of his realm's Earth.
Point of View - An alternate Samantha Carter and Charles Kawalski step into our realm, requesting assistance from an oncoming Goa'uld attack in their universe; General Hammond orders the mirror destroyed.
Ripple Effect - Dr. Lee confirms that General Hammond indeed ordered the quantum mirror destroyed, preventing them from now returning the SG-1 units from returning to their own realities in that manner.