The subspace capacitor was a power generation system created by a parallel version of the Atlantis expedition.
The capacitor was a new development in energy production that provided a "near-Zero Point Module level of energy", though it is unknown what exactly is meant by the term 'near'. It draws power from our subspace in a similar but non-identical manner to a ZPM, which draws power from an artificially created and contained region of subspace-time. Also, while a ZPM will eventually run out, a subspace capacitor is able to store power from our subspace-time, and thus is a renewable energy source. However, there is a finite limit to how much power can be stored and made available at any given time. So while there is great potential in this new energy source, it is limited at the moment.
This system was installed in a parallel-universe version of the Daedalus in order to provide power to its alternate reality drive. However, a flaw in the system of the drive itself caused the drive to constantly activate and thus jump from one reality to another once it gathered enough energy to initiate the action. (SGA: "The Daedalus Variations")
It is unknown if the "real" Atlantis Expedition experimented with the technology, but given its enormous energy potential, it is likely.