Posted 17 September 2009 - 01:15 AM
I'm not entirely sure I know exactly what you're asking.
In terms of near universal acceptance among players (hardware and software) you probably want to stick with MP3. Itunes does primarily sell AAC files (with an .m4a extension), which play fine on Ipods. The AAC files may, or may not, come with DRM copy protection. Unprotected songs can be converted to MP3 (at 128 kbps, a "good enough" bitrate) within Itunes pretty easily by right clicking on the song from within the Itunes program and choosing "Create MP3 version" from the options. Unfortunately, I don't use Itunes often enough to know if there's an easy way to tell beforehand whether a file is protected or not.
Probably the best known competitor to Itunes is Amazon. Amazon sells songs as MP3 files, mostly at bitrates that are higher than (better than) 128 kbps. Most songs are priced at $.99 (my experience is based in the US) with albums usually discounted and there are usually some free promotional songs.
There are plenty of netlabels that offer free and legal MP3 downloads, but these are usually going to be in fairly non-commercial genres -- electronic, dance, techno, etc.
There are plenty of live concerts available for legal downloading at Archive.org, in the live music section.
There are also services that sell MP3 files less expensively, like emusic and Amistreet. These are not endorsements -- read any agreements or contracts carefully and make sure the service offers enough music that you're interested in to make it worth your while. And, of course, you can buy from a variety of sources.