WinMX is alright. I prefer using Shareazaa, it‘s fairly new but comes with features designed to stop corrupted downloads. It also functions on 3 seperate networks meaning you can find almost anything you need. You can download it here:
http://www.shareaza.com/?id=download
and to answer the initial question, software such as Kazaa will never become pay-per-use. it‘s designed specificaly to run without the use of a centralized server, so it‘d be next to impossible for anyone to keep track of when you use it and then charge you for it. it would require a major software redesign, and if it did happen, you could always keep using the old version for free.
it will also never go the way of Napster, because once again, it runs without central servers. Therefore, nobody can shut it down without forcing every single user to delete his/her version of kazaa.