Another hit by Steve Pavlina, go read it.

Consider my current business model. I have no products, no inventory, no customers, no sales, no employees, and no office outside my home. I haven’t spent a dime on marketing since I launched this site in October 2004. But I earn about $40K per month, mostly from joint-venture promotions, advertising, affiliate programs, and donations. Two years ago this site was bringing in about $150/month, and one year ago it was earning around $6K/month, so that’s a pretty nice rate of growth. The income does fluctuate from month to month, but the positive cashflow is high enough that the fluctuations don’t matter. I maintain a substantial cash reserve too, so I could survive a very long time even if all my income suddenly shut off. This is much less risky than having a job.