Men Working in U.S. government's Weather Office, 1926. Source: Library of Congress

The Asbury Park Press has a freely accessible database of federal employees salaries at http://www.app.com/apps/pbcs.dll/section?Category=DATA.

Not all federal employees’ salaries are available.  As Ralph Smith of FedSmith.com notes, the database

does not include employees involved in security work, the FBI, CIA, Defense Department, nuclear materials, IRS, and jobs essential to national security are excluded from the database.

Nor does it include employees of the legislative branch agencies (e.g., the Congressional Budget Office, Government Accountability Office, and Library of Congress) appear in the database. (Congressional staff salaries are available at http://www.legistorm.com/.)