Ooh, I love Philip Glass's "Violin Concerto": http://www.youtube.com/watch?v=WB0wHIC0NS0

For sheer "holy crap I didn't know you could do that with a violin", you can't beat Sarasate's "Zigeunerweisen": http://www.youtube.com/watch?v=xir-5oAWxXE

And I loooove this piece called "Les beautés du diable," by François Dompierre. The cello in this is gorgeous: http://www.youtube.com/watch?v=bKGKbiWK58g&feature=related