There's never been a more exciting time to apply for careers in the United States government. It's also never been so easy with the creation of the new Pathways Program.
Here are some links to start your job search:
www.usajobs.gov the official site of the federal government; you can search for openings by agency, salary, etc. You can find jobs at NASA, the Peace Corps, the Department of Justice, Labor, Agriculture, etc.
www.policyjobs.net lists research-oriented policy jobs; must register to access some of the postings, but not for most of the entry-level positions. This is not necessarily working directly for the government, but is closely related to government issues.