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 on file-swapping services such as Napster, 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.