Redis - Redis is an open source in-memory data structure project implementing a distributed, in-memory key-value database with optional durability.
Polipo - A small and fast caching web proxy (a web cache, an HTTP proxy, a proxy server).
memcached - High-performance, distributed memory object caching system
ExaProxy - ExaProxy is a a high-performance non-caching proxy.
MongoDB - MongoDB (from "humongous") is a scalable, high-performance NoSQL database.
Privoxy - Privoxy helps users to protect their privacy.