419 scams. They are known internationally as "4-1-9" fraud after the section of the Nigerian penal code which addresses fraud schemes.
This is a great and very funny site where they go scam biating.
http://www.419eater.com
If you have a few hours free, then go to the letter section where you will find the corospondence between the scammer and his "Mugu" (fool) They take a while to get going as the Mugu has to build up a level of trust with the scammer. But once the scammer belives they are going to get the money it is amazing what you can get them to do. As I say you will need a fair bit of free time. But it is worth it. I really like this one.
http://www.419eater.com/html/joe_eboh.htm