CIAO - Columbia International Affairs Online - is a continually updated collection of working paper, books, journal articles, policy briefs, etc. It is searchable and browseable. Most of the documents are in .pdf format. Politics, especially matters related to international relations, is the main focus.
To get an idea of the materials you can expect to find in CIAO, see this recent list of New @ CIAO titles.
Link to the CIAO homepage.