HAProxy is a free, very fast and reliable reverse-proxy offering high availability, load balancing, and proxying for TCP and HTTP-based applications. It is particularly suited for very high traffic web sites and powers a significant portion of the world's most visited ones. Over the years it has become the de-facto standard opensource load balancer, is now shipped with most mainstream Linux distributions, and is often deployed by default in cloud platforms. Since it does not advertise itself, we only know it's used when the admins report it :-)

The HAProxy core team maintains multiple versions in parallel. Since version 1.8, two major version are emitted every year. The first digit usually indicates a breaking change (config format etc) but in practice rarely changes. The second digit indicates new features. Both constitute a branch. One extra number appears after these digits to indicate the bug fix release.