There is a group of drum players, called La Bomba de Tiempo, who play sold out shows every Monday night.[youtube=http://www.youtube.com/watch?v=5SeMMpTmXIE]
It is almost always held at the Konex Center which is a pretty cool outdoor venue where the group plays on these stairs which serves as the stage.
Each show is improvised and tons of people gather to dance frantically nonstop for hours, including tourists, old men, hippies, the rich; everyone.
There is a lot of hype for these shows and any young person in Buenos Aires seems to have been at least once.
Having been there twice now, I can say its definitelly worth going.
What else are you going to do on a Monday night?