I'm still slowly getting through all the songs posted in here, but before the thread got buried too deep, I wanted to contribute my favourite version of a song previously mentioned... Hall of the Mountain King... as performed by Apocalyptica:
On a much softer, yet similar note:
I won't repeat others, but agree that Beethoven, Tchaikovsky, Wagner, Mozart, Schubert, Philip Glass... all amazing, amazing, amazing.
On the piano, I love William Joseph's music (notably, and I figure relevant to this forum: he used to play the organ for the Phoenix Coyotes.)
(Go to ~0:22 seconds in to skip the woman talking)
Lastly, the LA Philharmonic (with conductor Gustavo Dudamel) has been doing a 3 part live broadcast of performances, shown in theatres across the States and Canada. The Beethoven and Tchaikovsky ones have already been done (I managed to go see the latter, and it was an absolutely delightful 2 hours) but in June, they'll be doing Brahms. If you like classical music–which, if you're reading this thread, one would assume you do–I'd highly recommend checking it out.
(More info, 'cause I suck at explaining things at 6:40 in the morning...
http://www.cineplex.com/Events/LAPhilLIVE/Home.aspx)