Well.... depends on what you want to accomplish.
Do you want to make money from advertising other company's product?
Do you want to advertise YOUR products on other sites?
Do you want to buy an adword so your site shows up when searching?
One of the CSB users that I work with has InvestmentTools.com
and uses every type of ad on his site possible at some point. He has had tower and banner ads with Google, Yahoo, MSN, amazon, and many others. He has people buy space on his site to advertise their products. He puts ads for his site on other sites.
He gets a decent traffic, has a simple click counter that he can report back to his advertisers.
Those advertisers support the site and pay for his advertising costs.
His site has been discussed on blogs and financial articles.
Those sites link back to his site, bringing in more traffic, so he can charge higher rates for advertisers...
He does not sell a PRODUCT. His SITE is the product and is totally free to all visitors.