• Stockhausen, Klavierstück XI for piano (see the score here; fuzzy—sorry)
• Berio, Sequenza I for flute (see an excerpt here)
• Pousseur, Scambi for tape (hear realizations of it here, and glance at the notes explaining how each performer realized the piece)
• Boulez, Third sonata for piano (see a performance here; note how the performer has to manipulate the pages of the score.)

Other works of possible interest:

Wolfgang Amadeus Mozart (attributed; 1792), Musical Dice Game

• Generate realizations of it to hear
• See parts of the score
• Video demonstration

Earle Brown, December 1952

• The full score (JPG)
• Compare several performances