The President's Budget of the United States for FY 2008 was sent to Congress on Friday. The total budget requested equals approximately $2.9 trillion, including $716.5 billion for defense. Use the Library of Congress Thomas database to view the status of appropriations measures back to 1998.
For information about the budget process see:
Budget of the United States: About
Searching guide from GPO Access
Wikipedia article on the United States budget process
A Glossary of Terms Used in the Federal Budget Process from the GAO (500 pages!)
The Budget Process from the House Committee on Rules