How to expand windows fully?
In OS X Yosemite, just click the green plus (+) icon while holding Shift (⇧) + Option (⌥) keys.

Or double click on the window's title bar while holding Shift(⇧) key (this won't work if System Preferences > Dock > Double-click a window's title bar to minimize is checked).

UPDATE

Here are proofs for those who voted down my answer before even trying if it works:


http://www.raymondcamden.com/2015/03/22/osx-yosemite-tip-maximizing-windows



  If you Google for this, you’ll see lots of people recommending holding
  alt-shift before clicking the green button.



Make the green full screen window icon on Yosemite maximize windows



  In order to maximize the window so that it fills the visible window
  content, use:
  
  ⌥ + Click on green icon
  
  In order to maximize the window both in width and height to the
  current desktop for applications like Google Chrome use:
  
  ⌥ + ⇧ + Click on green icon



How to change the default behaviour of the fullscreen/maximize button?



  ...by doing:
  
  ⌥ + ⇧ + Click on green or outer right icon (plus
  icon)
  
  I can maximize a window in both width and height to the current
  screen.


I tried it and it works on my MacBook Pro 13" Retina with Mac OS X Yosemite.

SUMMARY

There is no need to buy or hack something to do the following things:


Fullscreen mode: click on green icon
Resize window to it's content: ⌥ + click on green icon
Maximize window to screen size: ⌥ + ⇧ + click on green icon