Well done to the owners of classicalmusicmobile, who've made available a load of old classic recordings which have fallen out of copyright for a mere euro on mp3.
I have to say that the service compares well with the likes of Pristine Audio, who also reengineer old recordings, though at a slightly more expensive rate.
Aside from the early 1950s Callas operas, Ferrier, Flagstead, and about 2 dozen great 1940s and 1950s Wagner recording, there are offerings such as the complete (later) Budapest quartet Beethoven cycle (unfortunately the later 40-50s Sony cycle which is felt to be inferior to their earlier recordings), an early Sutherland BBC Weber Euryanthe, two magnificent Russian Tchaikovsky offerings (which even at their normal prices are great value). I'd certainly recommend this service over the Opera d'Oro series of old live recordings, many of which are very hit and miss.
A lot of the same recordings appear on Naxos Historical, although not available in the US due to the longer length of copyright cover. It should be very productive over the next few years, as a great many of the late 1950s stereo recordings date from the 1957-1960 period, so this is an area likely to see huge growth as these recordings fall outside the copyright (and indeed, why should consumers continue to be charged so much of a premium for downloadable formats?)
You can find this site at this link.