Seal Team 6 was founded in 1980 by Lieutenant-Commander Richard Marcincko. It's interesting to note that when Seal Team Six was created there were only two actual SEAL Teams. Marcinko allegedly named them this way to add the illusion of multiple SEAL units. In 1987 the name was later changed to "United States Naval Special Warfare Development Group," or DEVGRU. Seal Team 6 was the special forces unit that killed Codename: Geronimo (Osama Bin Laden) during an assault on his building on May 1st, 2011.
Seal Team 6 has been in Counter-Strike since Beta 1.