"Hammerklavier" is Ludwig van Beethoven's 106th Opus: Piano Sonata No. 29 in B flat major.
Though not one of Beethoven's most famous works, Hammerklavier is arguably his most brilliant. I think that what makes it so appealing is the palpable element of human emotion. When this piece was written in 1818, Beethoven was going through a period of great personal turmoil; he was in the midst of a battle with his brother's widow over custody of her son, Karl. Beethoven (who was given to occasional suicidal urges) had seized upon the idea that Karl was his last chance at having a normal social life in spite of his deafness. Hammerklavier juxtaposes his turbulent emotions against the heroic style that is typical of Beethoven's middle-period compositions.
When Hammerklavier sonata was first performed, there were some who declared it to be the end of music. I would argue that it was in fact the beginning of auditory thought. Beethoven once said that there are ideas which cannot be expressed in typical semantic vocabulary, but only through music. Hammerklavier is such an idea.
Hammerklavier Sonata can be downloaded from: http://www.ibiblio.org/nassar/beethoven-29-3.128.mp3