Proximity is in function somewhere between http-proxy and proactive-mirror. One of it's primary use is as Java web application to serve as Maven proxy on our company's intranet. As for reducing outgoing traffic (caching central and other maven repos), aggregating more repositories (reducing project config) with acting as one logical repository and for publishing in-house and other external maven artifacts which are not uploadable to "central" (like commercial projects, J2EE Jars, etc...).

This project is reached end of it's lifespan, and Sonatype Nexus is the successor of the good old Proximity1.