Redundancy, load-balancing, and failover are some of the most advanced features of modern networking. They are generally only necessary or required within large or sensitive systems and not all firewall and router products support these types of configurations. pfSense, of course, supports them all.
Redundant WAN interfaces (multi-WAN) provide a single firewall with multiple independent connections to the Internet. pfSense can then be configured to load-balance or failover the multi-WAN interfaces. Load-balancing would divide all traffic among the interfaces while failover would use a single interface, but upon failover it would automatically switch to another.
The pfSense load balancer allows for specific types of traffic (for example, web traffic) to be distributed among multiple servers. The ability to create your own webfarm is built right into pfSense!
Redundant firewalls allow the system to survive the death of a physical firewall machine. Using a CARP configuration, pfSense...