CBO (Congressional Budget Office) and the GAO (Government Accountability Office), they provide a service enabling members of both Houses of Congress to improve the quality of their scrutiny - a key role for legislators.
In the UK we have the House of Commons and House of Lords Libraries. The first is the best resourced of the two - and yet its counterpart in Congress - provides an excellent service to legislators. It's reports (Research Papers and Standard Notes) are publicly available here. In addition they provide an individual service to MPs - answering questions and providing specific research. The House of Lords Library is a smaller operation. It's Library Notes can be accessed here.
The National Audit Office carries out similar functions to the GAO. A description of their work can be found on DirectGov. "The National Audit Office works on behalf of Parliament and the taxpayer to hold government to account for the use of public money and to help public services improve performance. The National Audit Office is independent of government and has comprehensive statutory rights of access to the bodies it audits." It works specifically with the Public Accounts Committee, but its reports are an invaluable aid for other select committees. The reports are available here.
Select Committees and Public Bill Committees are assisted by the Scrutiny Unit, which consists of a small number of specialists. They are also useful in advising committees about external sources of specialist advice. Their reports can be accessed here.
The reports mentioned in this post are of course important for the work of Parliament and Congress - but they are publicly available - and I have found them to be useful for background reading - and for specific issues which I am researching. They can be very useful tools if you are a student, researcher, or wish to be one step ahead of others in your knowledge of particular subjects.