XNap is a plugin-based filesharing client written in Java, which makes it possible for you to run it on any platform that provides a Java Runtime Environment whose version is higher or equal to 1.3.0. (By the way: Mac OS 9 unfortunately does *not* provide such a Java environmnent, so XNap cannot be used on this platform.)
Plugin-based means that in theory XNap is able to use several different filesharing networks at once.