PSOne Classics are becoming a staple of the PlayStation Store, with new (old) titles releasing fairly frequently here in the States — but even more so in Japan. In fact Sony has announced that over on the Japanese Store, there are over 600 titles in the games archive.
Not all of these games are in fact PSOne Classics as you can also find TurboGrafx-16 and SNK Games but the bulk of the games come from that good old PlayStation One. Hopefully we will one day see the same amount of old school games on the U.S. side of the store.
[Source]