I am in the process of tuning a large distributed system; using an F5 BIG-IP Load Balancer to distribute traffic.
Willy Tarreau has a very good overview of load balancing options:
Making applications scalable with Load Balancing
Copyright © 2006-2008 Corey Goldberg
Disclaimer The opinions expressed herein are my own personal opinions and do not represent my employer's view in any way.