Varnish Cache is a web application accelerator also known as a caching HTTP reverse proxy. You install it in front of any server that speaks HTTP and configure it to cache the contents. Varnish Cache is really, really fast. It typically speeds up delivery with a factor of 300 - 1000x, depending on your architecture.
ocsp_proxy is a caching ocsp proxy. It accepts ocsp requests from any client, e.g. an ssl-webserver, and forwards the request to the corresponding ocsp responders or returns the ocsp response from cache. Can be used to mitigate unreliable ocsp responders that are, as required by murphy's law, always down when needed.
Dante is a free implementation of the proxy protocols SOCKS version 4 and SOCKS version 5 (RFC 1928) that supports both IPv4 and IPv6. It can be used as a firewall between networks and to control outgoing or incoming traffic. The package consists of two parts: a SOCKS server and a proxy client that supports SOCKS, HTTP proxies, and UPnP. RFC 1961 (GSSAPI) is supported in both the client and the server. Commercial support is available.
Burd's Proxy Searcher is a Windows GUI to easily uncover public and free-to-use or anonymizing proxy servers. It's based on a customizable and filterable search engine, and automatically probes result lists for server accessibility. It can test for HTTP or SOCKS proxies, and instantly apply settings to the current web browser.
SQL Relay provides a database proxy connection pool, load balancing, query routing and filtering. It eases replication and scalability. It supports MySQL, PostgreSQL, SQlite, ODBC, Oracle, Sybase, DB2, Firebird, MS SQL Server and MS Access. There are drop-in MySQL and PostgreSQL libraries to use it without client adaption. A native SQL Relay client API is also available for C, C++, C#, PHP, Perl, Python, Ruby, Java, TCL and Erlang, or drivers for Perls DBI, Pythons DB, Rubys DBI or PHPs PDO and
JAP / JonDonym is an anonymizing proxy service. It runs as local service connecting encryptedly to centrally shared proxy cascades. This hides the individual IP addresses of its users. With JondoFox there is even a simpler to configure Firefox plugin.