I read an article at the money.cnn.com site about Kevin Ham who owns a over 300,000 domain names and is portfolio is worth over $300 million. Most of that was built off of grabbing domains with mistyped names, those that people easily misspell.
Another avenue he is used is making deals with countries to use their domain name extention such as ".cm" which belongs to the West African coutnry of Cameroon. When someone makes a mistake and ends a domain name request with ".cm" instead of ".com", they are redirected to his site where the person is shown advertisements. He gets a cut of the revenue and so does the country.
Now he is moving into building out sites. He realizes there is far more money in developed sites than just the ad-clicks of the typo'ed sites.
You can read the interesting article at:
http://money.cnn.com/magazines/business2/business2_archive/2007/06/01/100050989/index.htm