Do you own an Apple iPod or iPod Shuffle? I personally think that subscription music services like Napster make more sense long-term than 99¢ downloads for each song if you are a rabid listener. If you are enamored with the fashion statement that is the iPod, then digital audio fidelity may not be your chief concern.
None the less, while I still believe that Windows Media Audio (WMA) is superior to Apple Audio Codec (AAC), I'm fairly certain I am not read much by Apple Acolyotes as I have not yet received much hate mail for my opinion.
Anyway, I started this post to tell you that you can move your purchased music around on your devices if you happen to use multiple players. My own player is an iRiver, and it plays Ogg Vorbis, WMA or MP3 (more on MP3 format). If I owned an iPod, I would probably go through the steps described by Bill Machrone of PC Magazine. In short, you sacrifice audio quality by burning your purchased music to CD, then ripping them back to your computer and putting them on your other music player.
If you are using iTunes, check out the Hymn Project which allows you to digitially convert your iTunes library to other digital formats, skipping the "rip to CD and back again" steps. This preserves your purchased audio quality.
Note that in order to do this and not violate any ethical standards imposed upon you via your religious or philosophical leanings, not to mention not violating the DCMA, you will need to purchase the music. And, you still cannot "share" these with your friends. That would be stealing for your friends and your would be distributing copyrighted material.