Published by OneSwitch.org.uk Saturday, 5 February 2011 10:26 am.
I could be wrong, I could be right, but the videos above are of what I think are the first ever commercial one-switch playable games. Kee Games/Atari's 1975 Steeplechase coin-op, and Fairchild's 1976 shooting gallery.
First ever one-switch computer game? No idea really, but the earliest I've seen is from the UK in 1969 on the computer nick-named "Nellie" (see top video around 2:38 minutes in). Probably the first audio game too. Ah, the good old days when you had to check the oil levels of your computer(!)
2014-03-14 Update: You can now play a version of the Nellie Reaction Tester on Windows PCs with huge thanks to William Pilgrim.