Most vim users are familiar with the man page lookup; K under the cursor or on visual selection. For anyone who needs a quick refrersher lets take a look at the help docs (help :K)

K     Run a program to lookup the keyword under the
      cursor.  The name of the program is given with the
      'keywordprg' (kp) option (default is "man").  The
      keyword is formed of letters, numbers and the
      characters in 'iskeyword'.  The keyword under or
      right of the cursor is used.  The same can be done
      with the command
        :!{program} {keyword}

So we can see that the default program (keywordprg / kp) is defaulted to "man" and the keyword is determined by what is right under the cursor when it is used. The other important thing to not is the fact we could invoke man or whatever program we want using :!program_name but that's not as fun as reconfiguring the default behavior to do what we want.

Let's imagine that for some reason we find ourselves copying sections of text and searching google for the results. Rather than doing this over and over why not just change the keywordprg to a custom bash script to do what we want. First thing's first lets write a simple bash script to open up the browser (I assume every OS has some way to open a browers with a given URL; This is written on an Ubuntu machine but if I were on a Mac I'd use open and test / google to make sure the syntax works as expected)

#!/bin/bash
firefox "https://www.google.com/serach?q=$1"

Give it a handy name like googleShit, move it into your PATH and and pop open that ~/.vimrc to change your default keywordprg

set keywordprg=googleShit

And now when you use K inside a new vim session you will be googling contents rather than looking up the man pages! If you find yourself repeating a task under the cursor or in visual mode pretty handy trick to have in the utility belt. Use a little imagination and you can come up with something to improve your daily workflow.