Many spammers are complete idiots…actually, most of them are. These spammers are a nuisance, which is why most of them come from ISPs that tolerate or even encourage spam. Taking these spammers down isn't easy, but it's a lot easier to block the entire spamming domain completely. On the other hand, some spammers are good at hiding their addresses. The ones using Asian servers, in particular, can hide themselves so that their IP addresses don't show up on many of the most common databases.
That's where this little utility comes in. Softnik's IP Lookup is a simple, powerful, and free utility that has successfully tracked every IP address I've given it. If you need to track down spammers, this is a good one to have.