The Spawn Control, Spawn Bones, and Spawn Null nodes will let you build the rig procedurally (automatically). These are incredibly powerful and can be used in a variety of ways in the construction event to create reusable rigs.
I get a lot of questions about the node Project to New Parent which is one of my favorite nodes. It basically lets you reparent transforms under rig elements, similar to the Parent Constraint or parenting a control to a bone. For the inputs, the Child input will be what you are parenting, and the Old Parent & New Parent inputs should be the parent and the same item.
One of my favorite special nodes is the Chain Harmonics, you can apply this to the root of a chain and it will drive those bones with a chain based harmonics system, in laymens terms adding automatic movement to the bones.
The Sphere Trace node will let you trace against level geometry, creating interesting setups such as tires colliding against the ground or foot sloping for characters to attach them to the ground.