If you'd like to be a startUp, lets look at two interesting services which you'll see quite large in the near future. I talk about swicki and edgeio. They aren't big companies with a lot of money, but they have something new to offer for web surfers.
If people find your web site valuable enough to place there something for sell, you can start to sell the space! Edgeio creates a mechanism for placing ads on your web site for money. Here is simple example: Techcrunch Board. The way of money is:
1. you show the price (you can distribute coupon codes)
2. somebody place an ad/offer paying to edgeio
3. you show the ad
4. edgeio pays you (by PayPal)
Simple? But effective :-)
Google is thinking about customising search results in that way. They already created a service for customising search results preferences. It is quite advanced.