Google Grants
Google offers free Google AdWords for nonprofits:
Google Grants provides eligible organizations with in-kind keyword advertising using Google AdWords so you can connect directly with your target audience. Through simple, short text ads that run on Google.com, thousands (or even millions) of people can learn about your organization online as they are searching for related information. When someone enters keywords (short phrases specifying a particular search query) into Google.com, ads targeted to those keywords appear alongside the search results.
For more information, go to the Google Grants page.