Another avenue of advertising revenue is image advertising. Most of the image ad networks pay for CPM (per thousand impressions), that means that these ads generate revenue not from click throughs like contextual advertising, but via ads being displayed. This form of advertising tends to pay less than contextual, but it is also a steady earner and reliable. A good piece of advice is to have display ads from two advertising networks on the same page. This way if one company is not able to furnish a particular page with advertising you have a backup with the other network. Two companies which provide such advertisements are ValueClick and Casale
Here I should mention that though there is money to earn by using pop-under advertisements, I think this is generally a bad idea. You have to consider your user base is sophisticated. Being interested in Open Source software is a filter, and your community is smarter than the average Joe. Most will be angered when exposed to such a thing.