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.
HAProxy is used by a number of high-profile websites including GitHub, Bitbucket, Stack Overflow, Reddit, Speedtest.net, Tumblr, Twitter and Tuenti and is used in the OpsWorks product from Amazon Web Services.