ModBackhand is an Apache module that provides load balancing and high availability features for a cluster of webservers.
Because it is part of Apache it works on a HTTP request basis (which also means it cannot be used for any other type of service, ie mail), this allows for smarter and more flexible distribution of requests. It also allows for features not found in lower level balancers like the affinity for servers based on user session, thus easing application development or clustering of existing applications that don't have a shared session system. ModBackhand is also cross platform, allowing almost all Unices (that run Apache) to do the balancing and also support for Windows based application servers.
ModBackhand allows you to also have the balancer on a machine which can also serve the actual requests, have it standalone on a machine with a stripped down Apache or even have multiple balancers for fault tolerance.
It works by the participating servers in the cluster broadcasting (Multicast can also be used for multiple clusters on the same network) stats about themselves (Load, CPU idle time, spare Apache processes, etc.) allowing for the balancer to make informed decisions about distributing requests instead of a naive round robin approach (such as DNS RR).
One benefit of ModBackhand over other LoadBalancing tools is that it's simple to setup and get a working cluster, it can also easily be combined with other tools to provide complete IP failover (Wackamole) or HighAvailability for logging (ModLogSpread).