By default, the Napster software connects to Napster's server, but this doesn't have to be the case. There are other non-commercial servers that fulfill the same coordination function of Napster's central server. Napigator enables you to choose among the ever growing group of Napster-like servers. Then one can use Napster in the usual way. The advantage of Napigator is that it is legally survivable.
Different servers have different advantages too.
Other servers may have fewer users, but are faster. Gnutella Gnutella has a rather interesting history. The software was briefly posted to the Nullsoft website before management took it down, but by then the genie was out of the bottle. Gnutella is versatile and invulnerable to legal action. Gnutella, on the other hand, uses a decentralized distributed server system.