I just noticed that Pactester has been released:
Pactester is a tool to test the proxy auto-configuration (PAC) files used by browsers to identify the correct proxy server for a given URL.
Since the PAC file evaluation mechanism is generated inside the browser and cannot be accessed from outside, the only way to tell which proxy your browser will use for a specific URL is manual inspection of the PAC file. Unfortunately, this approach is error prone and quickly becomes impractical for large and complex PAC files. Enter pactester, which resolves this issue by simulating browser behavior.
Normally I wouldn’t think much about this, but I am seeing PAC come up more and more, normally surrounding doing offline/sync work for Ajax applications.