I imagine you have heard of the band Dream Theater. The guitarist (John Petrucci) and keyboardist (Jordan Rudess) did a live, (mostly) acoustic album together called An Evening With John Petrucci and Jordan Rudess. As you can probably imagine, most of the songs are rather difficult to play. Regardless of your skill level, I highly recommend listening to this to either get some great songs to play or setting some high goals for yourself.
Here's one of the tracks: