Superconductivity is the transmission of current with no resistive losses, and is one of the most active areas of condensed matter physics research.
Superconductivity was first observed by Kamerlingh Onnes in 1911 when he discovered that the resistivity of mercury went to zero at 4.2K. Since that first discovery, superconductivity has been found in a wide variety of material classes, from pure elements to complex structures like cuprates, organometallics, and heavy fermions. The current record for high temperature superconductivity is close to 150K, which is found in a cuprate compound at high pressure.
Important outstanding questions in superconductivity research remain the origination of the pairing mechanism for unconventional superconductors, and whether room temperature superconductivity is possible.
There are many web resources for superconductivity. General introductions to the topic may be found in the following web sites: Hyperphysics: http://hyperphysics.phy-astr.gsu.edu/hbase/solids/scond.html Wikipedia: http://en.wikipedia.org/wiki/Superconductivity
The current research edge of the field may be found on the arXiv preprint server, which has an entire subsection devoted to superconductivity research: http://arxiv.org/list/cond-mat.supr-con/recent