Surge for Mac - Advanced Web Debugging Proxy for Mac & iOS
Polipo - A small and fast caching web proxy (a web cache, an HTTP proxy, a proxy server).
Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy
ExaProxy - ExaProxy is a a high-performance non-caching proxy.
HTTP Headers - HTTP Headers allows you to quickly see the HTTP header information for the current URL.
Squid Proxy - Website Content Acceleration and Distribution. Thousands of web-sites around the Internet use Squid to drastically increase their content delivery. Squid can reduce your server load and improve delivery speeds to clients.