Apple uses AAC files that only work with the iPod and Sony uses Atrac3 that only works with sony players. Real uses Real Audio 10 which didn't work with anything but now they introduced Harmony which makes Real Audio 10 compatible with many devices (see the Real website). All they other services use WMA files that are pretty much compatible with any player (not iPod) since most players support WMA that are protected. Also, Real Rhapsody is a streaming service so you can't download songs but you can pay .79 to burn a song to a CD then you can rip the song onto your computer.
Other than Apple/Sony's proprietary music files, can I download from Real or Rhapsody or any other pay-service and play them on any 128 or 256 MP3 flash drive?