HAProxy(High Availability Proxy) is an open-source load-balancer which can load balance any TCP service. HAProxy is a free, very fast and reliable solution that offers load-balancing, high-availability, and proxying for TCP and HTTP-based applications. It is particularly well suited for very high traffic web sites and powers many of the world's most visited ones.
Since it's existence, it has become the de-facto standard open-source load-balancer. Although it does not advertise itself, but is used widely. Below is a basic diagram of how the setup looks like: