HAProxy is a free, very fast and reliable solution offering high availability.
EditBy: 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.

Additionally, a third version 1.3 is under active development. New features include :

* Content Switching : provides ability to select a group of server based on any part of the request such as the URI, the Host field, cookies, or anything else. There is a growing request for this feature from large sites which separate dynamic and static contents.
* Full Transparent Proxy : it is possible Connect to the server with the Clients IP address or even any other IP address. This is possible only on Linux 2.4/2.6 with the cttproxy patch. This feature also makes it possible to transparently handle part of the traffic for a particular server without changing any servers address.
* New FASTER tree-based scheduler : versions up to 1.2.16 required that all timeouts were set to the same value to support tens of hundreds of connections at Full Speed. With this new scheduler, it is no longer the case. I have backported it to 1.2.17.
* Kernel TCP splicing : avoiding kernel-to-user then user-to-kernel data copies improves bandwidth and lowers CPU usage. Haproxy 1.3 supports Linux L7SW in order to achieve multi-gigabit performance on commodity hardware.
* Connection Tarpitting : since the cost of maintaining a connection open is low, it is sometimes desirable to tarpit attack BOTS, which means maintain their connections open to limit their Capacity. This has been developped for a site crawling under a small DDoS with easily identifiable requests from a few thousand zombies, and has since saved several sites. you can free download HAProxy 1.3.15.7 now.