National Association for Business Economics is an excellent web site for analysis of economic policy.
http://www.nabe.com/publib/index.html

Congressional Budget Office is an excellent source for data on the federal budget, how the budget system works, and the lastest budget information.
http://www.cbo.gov/index.html

Economic Policy Institue as the title suggest, a web site dedicated to the study of economic policy.
http://www.epinet.org/

A Citizen's Guide to the Fendera Budget an online copy of the assigned text. Useful for getting charts and graphs to incorporate into policy memos and essays.
http://www.whitehouse.gov/WH/EOP/OMB/html/bud96gui.html

Re-inventing America online survey of how Americans would modify the current federal budget. Extremely useful for gauging public opinion on budget items.
http://www.crossover.com/reus/index.html

Center on Budget and Policy Priorities as the name suggests, policy oriented web site dealing with budgetary issues.
http://www.cbpp.org/

CTJ Reports is a web sited focusing on taxation and budgetary management. Has good data sets, and regional tax information.
http://www.ctj.org/html/reports.html

National Budget Simulation is a web sited which allows you to modify the federal budget, to demonstrate how difficult it is to balence the federal budget.
http://garnet.berkeley.edu:3333/budget/budget.html