121.500 and 243.000 are the standard guard frequenies. Since civilian aircraft only has VHF voice capability, they monitor 121.5 on a regular basis. It is also used in distress and/or lost contact with ATC. All aircraft is encouraged to monitor the guard frequency at any time or when workload permits.
Also, 123.45, 234.5 and 345.6 is another set of frequencies used over the ocean as a "Air to Air" channel, but less formal. Think of them as like a "party line"
