A supposed "peak" in programming ability caused by alcohol. The term was coined by an XKCD web comic: http://xkcd.com/323/. The phrase is in reference to Microsoft CEO Steve Ballmer, who generally acts pretty crazy, much like an angry drunk, and hence the alcohol joke.
Somewhat ironically, Ballmer never wrote any production code while at Microsoft. His background is in business rather than engineering, though he is renowned for his ability to perform difficult mathematical calculations mentally.
I finished that programming assignment in one night thanks to the Ballmer Peak.
Man, they really must have overshot the Ballmer Peak when they wrote Vista.