Greg - SiteOrigin

I built this plugin to help ease the transition during a DNS changeover, but you can use it any way you please. It turns your WordPress installation into a very simple proxy server. It fetches content from another URL, caches it and displays it. Requests are cached for 24 hours.
Install the plugin, then head over to Settings > Simple Proxy in your WordPress admin. Enter the URL you want to proxy to and when you’re ready to go, also check the enabled checkbox.
You need to have <a href="https://codex.wordpress.org/Using_Permalinks#Choosing_your_permalink_structure" rel="nofollow ugc">pretty permalinks</a> enabled.
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Simple Proxy

Turns your WordPress installation into a simple caching proxy.

sleepm

Manage WordPress proxy in one page, you can set some domains that can be accessed through proxy. 
Also, http basic auth support.

WP Proxy

Manage WordPress proxy in one page, you can set some domains that can be accessed through proxy. Also, http basic auth support.

Simple Proxy vs WP Proxy