It's about time that the world of online music downloads expanded to include a greater emphasis on genres other than pop, so I'm surprised and delighted to see this - the BBC is offering Beethoven's complete symphonic works for free download in MP3 format. But you're going to have to snap them up quickly - each symphony will only be made available for a limited period.
One notable exception to the rather barren selections offered by iTunes and their ilk is Magnatune, of whom I've become a great fan in the last few weeks. Their selection continues to grow and they're also starting to attract musicians of the calibre of Trevor Pinnock, which can only enhance their reputation. They have some great gems and hidden surprises; I've enjoyed everything from Eastern Orthodox sacred music to instrumental electronic space pop (now there's a niche!). Give them a try - they operate on something of a shareware model and are the kind of company you feel good about giving your money to.