From the README: "Libproxy is a simple rewriting pass-through proxy system designed especially for libraries... If you have patrons who are off-site, and who would like access to your on-site-only (e.g., IP-restricted or passworded) journals, databases, and full-text retrieval systems, then Libproxy is your solution." Get the latest source here or check out the overview online (it's also bundled with the code).