For relaxing most of Bach and Brahms are soothing and relaxing. But you also mention music to work to--you don't want to fall asleep (or maybe you do)--but I would recommend Mozart's work for that. There have actually been studies that showed listening to Mozart raised one's IQ a few points.
More Dumb "Classical" Stereotypes
As little as I know about "classical" music (in the Tower Records sense of the word) even I get bored and offended to hear people say "I LOVE "classical" music -- it's so RELAXING!!" Isn't more baroque music is played in coffee shops and dentist offices to "relax" people? I find Stravinsky quite fascinating and wonderful, but certainly not "relaxing." Whatever the distinctions or period confusion I have, I would love to hear your thoughts and recommendations for "classical" music that you find relaxing and conducive to concentration and/or happy "classical" music that fills you with energy and optimism. Obviously, a great recording would be an added bonus.