I've probably ranted about this before, but peer-to-peer scam sites piss me off.
Basically, they're sites that trick you into thinking they're legal download services - they then charge you a nominal fee (sometimes one-time, sometimes monthly) and give you a freeware p2p app and some crappy documentation.
I've found a really helpful reference site with regard to these scams: p2pscams.info. For most people with a reasonable level of Internet and p2p literacy, it won't be a problem to discern between what is and isn't a scam site (in short, there are very few legal download services, and most of them are reasonably well-known such as iTunes, Napster, Yahoo Music, etc). But this site is a helpful reference to hand out to people who aren't as computer-literate as you.
It explains exactly how the scams work (basically, they are able to stay relatively legal by just having in the fine print "you should ensure that you have permission of the copyright holder before downloading any copyrighted material over a p2p network"), and has a fairly comprehensive list of scam sites. They're arranged, basically, by what they spoof as, for example, the LimeWire section currently includes 9 different LimeWire scam sites as well as linking to the official LimeWire website.
A useful reference, especially if you don't have the heart to tell someone that they are downloading music illegally and are paying some scammer $40 a year for the privilege.