Well there's the old joke that the best way to make a small fortune on the internet is to start with a large fortune!
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Sell Something. A product or service.
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Do advertising. Ie Have a site that draws lots of people (free dating?) as you need lots of traffic to make money from adverts
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Launch your own game (think Zynga on Facebook but thats needs fairly specialist knowledge).
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Write for an online publisher. Fine if you can get it but it's hard work like everything else.
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Gambling. Probably a good way to lose money unless you are very good eg Playing online poker.