It depends on what you consider to be motivational, but I'll take a shot at it. These may be more inspirational than motivational, but I suppose the two are related.
I listen to a lot of music from games and Chrono Trigger has some of the best. Check out a short clip of
Play! A Video Game Symphony performance of Chrono Trigger music. A longer video is
here. Also,
World of Warcraft has some great orchestral music.
I can also recommend
The Terminator theme. Star Wars has some great music too. While I'm in sci-fi -- Star Trek: The Next Generation opening theme is good, and also
Babylon 5 opening theme. I linked to Amazon samples of some of these.