Congressional Budget Office
CBO's mandate is to provide the Congress with
Objective, nonpartisan, and timely analyses to aid in economic and budgetary decisions on the wide array of programs covered by the federal budget and
The information and estimates required for the Congressional budget process.
Federal outlays through 2016, long term outlook on Social Security costs, the costs of disaster like Katrina and war engagements like Iraq, are just a few of the many data sets available, often in both pdf and excel format.
Link to resource:
Congressional Budget Office
Fulltext available: Government Documents