Lurking below the sheer mass of purchasable MP3 sites like iTunes and Napster is the quietly growing Netlabel movement. Netlabels are simply online record labels that give their content away for free. In raw defiance of the motto "you get what you pay for", some of these operations actually offer some up some really really good stuff and a modicum of exploring can quickly yield a payoff of new and free music that you actually like.
Where better to begin this foray into the realm of costless, DRMless tunes than Archive.org's Netlabel Archive? Broken up by genres, this archive has hundreds of links to sites that thrive outside of the music business model.
Two stellar examples of sites that offer top quality audio are Thinner/Autoplate (Dub, Tech House, Ambient) and Tokyodawn (Downtempo, Hip Hop, NuJazz).