Pound. The Pound program is a reverse proxy, load balancer and HTTPS front-end for Web server(s).
Pound was developed to enable distributing the load among several Web-servers and to allow for a convenient SSL wrapper for those Web servers that do not offer it natively. Pound is distributed under the GPL - no warranty, it's free to use, copy and give away. Pound is a very small program, easily audited for security problems. It can run as setuid/setgid and/or in a chroot jail. Pound does not access the hard-disk at all (except for reading the certificate file on start, if required) and should thus pose no security threat to any machine. Don't use Pound for load balancing - Optimize Prime. We were using Pound for load balancing at Justin.tv until today.
It was consistently using about 20% CPU, and during spikes would use up to 80% CPU. Under extremely high load, it would occasionally freak out and break. We just switched to Ngnix, and load immediately dropped to around 3% CPU. Our pages feel a little snappier, although that might be my imagination. Crossroads load balancer program homepage.