While you'll likely start off with one or more of the established ad networks, as your site grows and builds an audience, you can also consider reaching out to advertising and media-buying agencies, as well as directly to brands (Nike, Pepsi, Ford, etc.) to place their ads on your site.
Suppose, for example, you have a site focused on health and fitness, with a large and loyal following of visitors who regularly come to check out your content. That could be an opportunity for a company like Gatorade to run ads on your site, take over the background wallpaper, or even run an online campaign with your site directly. Of course, this is the hardest type of advertising to get, and requires you to make a significant effort in outside sales. But the revenue could be quite high, and the relationships you make could be profitable for a long time to come.
Remember: When you own a website, and your primary source of revenue is selling advertising, you're basically a media company, like any newspaper or magazine. To make the most money possible, you're going to need to create some compelling content and put forth a real sales effort, including the development of comprehensive media kits and rate cards, to get brands to market against it.