There is always Google ad sense, but, what if you want to sell even more ad space?

AdBrite is a way for publishers to add revenue to their site.

AdBrite – How AdBrite Works
AdBrite’s aim is to provide our publishers with the most control, best service, and highest payout possible.
Step 1 – Choose Products

Through a single snippet of HTML, AdBrite can serve any combination of the following ad formats:

* Text and banner ads with customizable layout
* Full page ads – A high-paying full-screen ad on the third pageview of your site, shown only once per user per day

Step 2 – Attract advertisers

Anyone visiting your site can buy an ad directly on your site by clicking “Your Ad Here.” You’ll also be listed in the AdBrite marketplace, and represented by our busy in-house sales staff.
Step 3 – Control

You can choose to review each ad before it appears on your site. You can set the price for ads bought directly on your site. You can even have AdBrite show another ad network (your Google AdSense ads, Burst, etc.) if we can’t meet your minimum revenue requirement.

For really high volume sites, you can look to other ad placement services like Doubleclick which is now also owned by Google.

One thing you have to be aware of: once you ad advertising from a remote server, your page load times can start slowing down, which can lose you readers and reach. Some really successful sites can sell their own advertising and serve the ads internally, but you have to have a really targeted audience that advertisers want to reach.