Mediamonkey is a great free all round package – very similar to itunes, but less proprietary
Show me the proprietary part of iTunes then. To save you the effort, there is none. AAC is MP4, the higher spec version of MP3, and a great many devices will play AAC/MP4. Of course, you could point out that both MP3 and MP4 are proprietary, and you’d be correct in that, but then Mediamonkey would be just as proprietary as iTunes, wouldn’t it? You might as well use iTunes, which is free, after all.