Below are some sites you should visit that list a bunch of nonprofit jobs. Unless otherwise noticed, all of these sites are free to use for job seekers.
-The Nonprofit Job Seeker: The NonProfit Times' Job board. Lists nonprofit jobs from across the United States. Contains jobs of all experience levels. Make sure to sign up for The Nonprofit Jobs e-newsletter.
-Foundation Center: Also focuses on nonprofit jobs in the United States. Pretty good interface, and easy to use.
-Exec Searches: Mainly focused on executive level nonprofit jobs. Good site to use if that's the kind of position you are looking for.
-Idealist: One of the more popular nonprofit job sites out there. Contains jobs of all experience levels.
-Nonprofit Career Network: Allows you to search for nonprofit jobs across the country. Can also sign up for a newsletter that will give you up-to-date info on career fairs in your area.