As I was editting a python script this morning, I acidentally entered K in normal mode. Vim quickly hid and revelead a shell which contained two lines:

no Python documentation found for 'd'

Press ENTER or type command to continue

What is this sorcery, I thought to myself. I quickly googled but couldn't quite find an answer. Then I remembered vim's built-in help system and it revealed the following:

K

Run a program to lookup the keyword under the cursor.

Cool, ain't it? It uses the program specified by the variable keywordprg, the default is man -s but for python files it changes to pydoc. That's something worth remembering.