You can also try Bigpond music (of you are with them) as they dont charge you download fees for the MB/GB of downloads for the music you purchase off their site. (unmetered is the term) You can do a search like Itunes and such and then simply pay for what you want using an account. Quite cheap, but not sure of the amount of content there.
Stay away from ANY torrent type/sharing programs like Limewire etc, you will get a NASTY bill if you leave it on overnight and other people start downloading off YOU, not to mention the spyware/viruses/botware etc that proliferate on these types of programs.
