Ultra Moneky is a load balancer similar to the likes of pound and haproxy. It allows multiple web servers to be seen as 1 by the client.
HAProxy is a TCP proxy / load balancer which will work for HTTP and a range of other applications. It advertises a small but expert user base and offers a range of very useful load balancing features. Apparently it competes with pound and ultramonkey.
An article describing a system that uses client side load balancing in a Web application. Voxlite is the example application built on Amazon EC2 and S3.