The 2008 Plum Book, which lists federal government positions appointed by the President, is now available online via GPOAccess. Officially know as the United States Government Policy and Supporting Positions, this publication is compiled every four years, following the Presidential election.
These are the major categories of positions, listed on p. iii:
• Executive Schedule and salary-equivalent positions paid at the rates established for levels I through V of the Executive Schedule;
• Senior Executive Service (SES) ‘‘General’’ positions;
• Senior Foreign Service positions;
• Schedule C positions excepted from the competitive service by the President, or by the Director, Office of Personnel Management, because of the confidential or policy-determining nature of the position duties;
• Other positions at the GS–14 and above level excepted from the competitive civil service by law because of the confidential or policy-determining nature of the position duties.
Even if you're not looking to try for a Presidential appointment, the Plum Book provides a wealth of information about those positions!