Pound

Pound Website: The Pound program is a reverse proxy, load balancer and HTTPS front-end for Web server(s). Pound was developed to enable distributing the load among several Web-servers and to allow for a convenient SSL wrapper for those Web servers that do not offer it natively. Pound is distributed under the GPL - no warranty, it's free to use, copy and give away.

WHAT POUND IS:

a reverse-proxy: it passes requests from client browsers to one or more back-end servers.
a load balancer: it will distribute the requests from the client browsers among several back-end servers, while keeping session information.
an SSL wrapper: Pound will decrypt HTTPS requests from client browsers and pass them as plain HTTP to the back-end servers.
an HTTP/HTTPS sanitizer: Pound will verify requests for correctness and accept only well-formed ones.
a fail over-server: should a back-end server fail, Pound will take note of the fact and stop passing requests to it until it recovers.
a request redirector: requests may be distributed among servers according to the requested URL.

From Wikipedia: Pound is a lightweight open source reverse proxy program suitable to be used as a web server load balancing solution... Using regular expression matching on the requested URLs, Pound can pass different kinds of requests to different backend server groups.