For FreeBSD guests, Virtualbox can sometimes report the private network interface IP address as "0.0.0.0". This will cause an invalid NFS exports file to be generated for FreeBSD and OS X hosts. Fixed by not allowing Virtualbox to report a guest IP address of "0.0.0.0".
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.