Perlbal - load balancing white paper
Thanks to a TPF Grant there is now actual Documentation which really makes using it a whole lot easier! I've had this in production for about 4 years now and it's very stable. Being able to change which servers (or ports) your entire sites traffic goes to make gradual rollouts (and quick rollbacks!) very easy. It also makes upgrading your servers far less complex.
If your not already using a web load balancer infront of your web server check it out!