In windows server 2003 there is a feature built into the TCP/IP networking and IIS that allows for network load balaning to happen between two servers, without dedicated load balancing hardware e.g, ...

The transport-level server load balancing architectures described in the first half of this article are more than adequate for many Web sites, but more complex and dynamic sites can’t depend on them.

Mod_proxy_balancer, an add-in module for the Apache HTTP Server, acts as a software load balancer to split traffic across multiple back-end servers.