Every web developer knows Fiddler. If not, you are missing out on some awesomeness. 

Sometimes, it would be nice to make Fiddler ignore certain ports on e.g. localhost. Currently, due to certificate issues, I would like to skip localhost:8081 and simply pass the response from this port directly rather than having Fiddler catch it.

This is simple, just Ctrl + r in Fiddler, and you get a Javascript called CustomRules.js. Look for the method called OnBeforeRequest. Here you simply add the following, and Fiddler will not process content from localhost:8081:

if (oSession.host=="localhost:8081"){
    oSession["ui-hide"] = "true";