Kurt Schwitters' Ursonate
Jaap Blonk has been performing this piece since 1982. He has it memorized.
Upon special request he is still willing to perform it.
A descriptive text by Jaap Blonk is here.
His recordings of the piece can be found here.
Also possible as 'Ursonography' with Golan Levin's animated typography of the full text:
Montage of video stills from Ursonography performance by Jaap Blonk and Golan Levin at Ars Electronica Festival, Linz, September 2005