Proxy Configuration Via "proxy.pac"
Overview
Many modern User Agents implement a method of configuring proxy servers based on the Netscape-designed "proxy.pac" feature, also known as Automatic Proxy Configuration (APC). This utilises the javascript engine to provide administrators with a method of allowing the User Agent to selectively uses proxy servers based on certain criteria.
Articles
Internet Explorer Caching - Microsoft Internet Explorer's automatic proxy caching; how it can interfere with proxy.pac load balancing and failover
Examples
Most modern graphical Web Browsers implement APC, including the current releases of Firefox, Internet Explorer, Netscape, Opera, and Safari.
Sun's Java Web Start (JWS) has limited support for APC.
Manu Garg has written a PAC-parser library with python bindings, available at http://code.google.com/p/pacparser/
Further Reading
Original specification: http://wp.netscape.com/eng/mozilla/2.0/relnotes/demo/proxy-live.html
Wikipedia overview: http://en.wikipedia.org/wiki/Proxy_auto-config
Microsoft proposed IPv6 extensions to proxy.pac: http://blogs.msdn.com/wndp/articles/IPV6_PAC_Extensions_v0_9.aspx