The House and Senate only just recently created new policies to require that their expenses (officially, the Statements of Disbursements) would be posted online. While we're still waiting for the Senate to start posting expense tracking, the House has been posting quarterly expenses since early 2010. Sunlight uses those expense reports to create a searchable database of expenses, and also to create a directory of House staffers.
Rep. Peters' bill (H.Res 135) would create a mandate for the Chief Administrative Officer to release expenses online (currently this process is based on a letter from Speaker Pelosi, and Speaker Boehner's continuation of the same policy). The bill would also have Member-specific reports posted on House.gov, and require each Member to create a link on their site to their reports.
This is a step in the right direction. We'll also be working for data-level access to the disbursements data (which is currently posted in a PDF that we parse), and more timely disclosure (than the once-per-quarter system we have now).