The Apache web server includes a complete HTTP/1.1 proxy.
The Apache web server was designed to maximise ease of programming — a decision which makes Apache immensely popular for deploying web-based applications. Of course, this ease of programming comes at a cost, and Apache is not the most lightweight proxy available.
As cheaper caching proxies are available, Apache is not useful as a standalone proxy. The main application of Apache's proxy is to join multiple web servers' trees into a single hierarchy.
The Apache proxy doesn't cache partial instances and doesn't pipeline multiple outgoing requests.