The only portable digital music device that limits you to a proprietry music format is the new sony player. Not a bright move on their part I feel, look what happened to Betamax.
The ipod uses mp3 which you can produce using several types of easily available software and you can also download ready made mp3 tracks directly and legally now from websites such as Apples Itunes at reasonable cost. There are many differnt mp3 players around now, many of them with immense storage capactity. Many of them also support WMA which is the microsoft music format which is supposed to compress files even further with equal quality playback, not used it myself though.
I use an ipod mini and it suits me perfectly, 1000 tracks where ever I go.