I believe these two videos are appearing on YouTube for the first time and have rarely been seen. They are two performances featuring Gary Burton and the Piazzolla Quintet at the Montreux Jazz Festival in 1986. The performances are of La muerte del ángel and Nuevo tango.
An audio recording of the full concert is available on a CD titled The New Tango.
To many Piazzolla fans, this is a legendary performance. I suggest you watch these two videos quickly. While these two videos may survive YouTube copyright surveillance some of the many other videos posted by the person who posted these many not and YouTube policy can shut down the entire account, taking these classic videos down with the others.
So, with that alert, click here for La muerte del ángel and here for Nuevo tango.
Note added on December 5, 2009: One more video from this Montreux performance has been posted. It is the final portion of the composition Laura's Dream.
To learn more about Piazzolla videos, visit the Piazzolla Video site.