Product: HAProxy - The Reliable, High Performance TCP/HTTP Load Balancer

Update: Load Balancing in Amazon EC2 with HAProxy. Grig Gheorghiu writes a nice post on HAProxy functionality and configuration: Emulating virtual servers, Logging, SSL, Load balancing algorithms, Session persistence with cookies, Server health checks, etc.

Adapted From the website:

HAProxy is a free, very fast and reliable solution offering high availability, load balancing, and proxying for TCP and HTTP-based applications. It is particularly suited for web sites crawling under very high loads while needing persistence or Layer7 processing. Supporting tens of thousands of connections is clearly realistic with todays hardware. Its mode of operation makes its integration into existing architectures very easy and riskless, while still offering the possibility not to expose fragile web servers to the Net.

Currently, two major versions are supported :
* version 1.1 - maintains critical sites online since 200 The most stable and reliable, has reached years of uptime. Receives no new feature, dedicated to mission-critical usages only.
* version 1.2 - opening the way to very high traffic sites

The same as 1.1 with some new features such as poll/epoll support for very large number of sessions, IPv6 on the client side, application cookies, hot-reconfiguration, advanced dynamic load regulation, TCP keepalive, source hash, weighted load balancing, rbtree-based scheduler, and a nice Web status page. This code is still evolving but has significantly stabilized since 1.2.8.

Unlike other free "cheap" load-balancing solutions, this product is only used by a few hundreds of people around the world, but those people run very big sites serving several millions hits and between several tens of gigabytes to several terabytes per day to hundreds of thousands of clients. They need 24x7 availability and have internal skills to risk to maintain a free software solution. Often, the solution is deployed for internal uses and I only know about it when they send me some positive feedback or when they ask for a missing feature ;-)

According to many users HAProxy competes quite well with the likes of Pound and Ultramonkey.