You can download a great variety of full works by many composers, including some excellent interpretations, for free, because the copyrights have expired. The site is called Musique Ouverte; it is in French, but you should have no problem navigating, even if you don’t know French. Here is a link to the search page, where you write the composer you are interested in, and you are presented with a list of available downloads. Other options include recently added recordings and links that don’t work anymore. You can also report dead links yourself.
If you prefer to listen rather than download, or if you don’t find at Musique Ouverte some work, visit the Deezer site, where you are allowed to listen even to copyrighted music, in full length but not in very good sound quality (128 kbps).