Napster essentially enables users to root around other people's hard drives in search of files.
Many activities are perfectly legal.
You can, for instance, copy a song you currently own, but only for personal use. Although copyright issues get most of the ink, security risks also exist. Napigator works in conjunction with Napster, but it lets you choose your own servers to poke around in, Napster or non-Napster. This has the advantage of letting you keep using Napster in precisely the same way you've been using it. Hotline is one of the best-kept secrets on the Internet, and it predates Napster.