The plug part is http://duckduckhack.com/ -- that is, these are open source components that plug into the system via this glue. This one in particular is at https://github.com/duckduckgo/zeroclickinfo-spice/blob/maste... and https://github.com/duckduckgo/zeroclickinfo-spice/blob/maste...

The in part is they get submitted and we try hard to make them all useful and thus "in" DDG by default. However, ultimately you could a) consume them in your own applications (off DDG); and b) choose to turn some off or on more of the time.