These are commonly called a 419 scam. If you want a good laugh, take a look at a couple of these sites:

http://www.ebolamonkeyman.com/ - This guy had a good time scamming the scammers. The site is unavailable sometimes, I don't know what the deal is with that.

http://potifos.com/fraud/ has an archive of a lot of these letters.

http://www.scamorama.com/ Another archive with some humor thrown in.