I usually enjoy mixing my own array of songs. That way it is stays original. :)
I believe Century actuall sells CD's of different types of music that are good for demos. http://www.centuryfitness.com/webapp/wcs/stores/servlet/CategoryDisplay?catalogId=10051&storeId=10051&categoryId=10051&langId=-1&parent_category_rn=13753&crumb=13501
It really all depends on the type of demo you are doing.
At the school I go to, we use music that tends to "build up power" in its sound, like, it gets stronger as the demo gets more intense.
Although we stay away from techno... but thats just my opinion.