CraftBook is a server-side mod that adds a long list of new gameplay mechanics to Minecraft, from automatic tree farms to automatic doors made of fence blocks. It's one of Minecraft's first mods, and it was originally written by us, but now it's maintained by Me4502.
You can find a lot of information on CraftBook on its wiki, but not all features are enabled.
We do not use most of CraftBook's rail mechanics; instead, we use our own.