As far as I know there isn't a linux version of Kazaa, at least there wasn't when I got back into using linux in september, and I haven't heard of one being released.
When you see people running it in linux it's probably through another program caled WINE (
www.winehq.com) which allows windows programs to run under linux.
However, don't take this as fact. If you do find a linux native version of Kazaa, please tell! :-D
As for running on an Amiga, you may be pushing it. I know you can get linux running on an amiga (I can look up the distro's name and website if you like) but I dunno if it supports PPC boards. If not, sorry, but forget it, unless your prepared to put a shed load of effort in to get a GUI up and running (you'll have to tweak X a lot to make it a runnable speed i'm told, or install an alternative to X). If it supports PPC boards, then you should be able to install it.