Besides MP3, the main formats in use are WMA/PlaysForSure from Microsoft, AAC (the DRM version is from Apple) and ATRAC from Sony, all of which are mutually incompatible.

For those who want an open-source option, there is the Ogg audio format, the format used by Wikimedia Commons. Ogg is the container format for Vorbis, Theora, Speex, FLAC, and many others.

A huge amount of freely downloadable music is available in MOD format from sites such as The MOD Archive[3]. Most MOD files are composed by individuals who allow them to be freely distributed, unlike music by most commercial artists. While today's digital audio players generally do not support MOD files directly, progress is being made on that front with projects such as the TRAXMOD digital audio player. Apple iPod owners willing to run Linux on their iPods may also play MOD files through the MikModule program. For the time being, MOD files can be manually converted into a supported audio format with some music converter and/or player software. It should be noted that due to differences in the file formats, the resulting file will almost inevitably be larger and of slightly lower quality.