haproxy

HA-Proxy is a TCP/HTTP reverse proxy which is particularly suited for high
availability environments. Indeed, it can:
- route HTTP requests depending on statically assigned cookies
- spread the load among several servers while assuring server persistence
  through the use of HTTP cookies
- switch to backup servers in the event a main one fails
- accept connections to special ports dedicated to service monitoring
- stop accepting connections without breaking existing ones
- add/modify/delete HTTP headers both ways
- block requests matching a particular pattern

It needs very little resource. Its event-driven architecture allows it to easily
handle thousands of simultaneous connections on hundreds of instances without
risking the system's stability.