Email SPAM. Nobody likes it, it's illegal in some countries, ISPs get very upset, recipients get even more upset but no list of how people make money online can be complete without a mention of this. What do you sell when you spam? Have a look at what type of spam YOU are getting, it will likely give you an idea of what pays. And the stock tips that seem to "accidentally" land in your inbox. But that's about all the help I'm giving you on the subject.
(But wherever there's a big problem there's an opportunity: Run an free email service like Hotmail etc., but with a good SPAM filtering system. High startup costs, sure, but if you get enough people signing up you could be worth a few million very quickly. Google tried this with gmail and got about 4% of the market without advertising the service at all; accounts were given by invitation only)