Quantum mirror

From The Stargate Omnipedia

Quantummirror.jpg
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.

SG-1 discovered a quantum mirror on P3R-233, and stored it at Area 51. After two unintended contacts with parallel universes, General George Hammond ordered the device destroyed.

KEY EPISODES

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.