Wikipedia
HAProxy
HAProxy is free, open source software that provides a high availability load balancer and proxy server for TCP and HTTP-based applications that spreads requests across multiple servers. It is written in C and has a reputation for being fast and efficient (in terms of processor and memory usage).
HAProxy is used by a number of high-profile websites including GitHub, Bitbucket, Stack Overflow, Reddit, Tumblr, Twitter and Tuenti and is used in the OpsWorks product from Amazon Web Services.