A simple load balancer such as Microsoft NLB is great for preventing downtime due to a failing web server, but as your websites on a server grow in number, you need something with more intelligence. Websites themselves may fail, as an application pool stoppage, without shutting down the entire server. In these cases an application layer (layer 7) load balancer such as Microsoft’s Application Request Routing (ARR) is an excellent choice for many web farms. ARR (which is free) can detect a failure to an individual website or application pool and then switch to a new website on a different server that’s functioning. ARR also provides detailed health monitoring and logging information along with easy management for a large web farm.