I actually used to carry one of these: 
   
  But I found - repeatedly - that I'd run into outlets like this:   
 
 At airports and outlets like this at client locations:   
 
  And the right angle plug would be problematic. 
So then I started carrying this: 
 
 But the 8' length was too much so I cut it to 4' and put this male plug on the cord: 
 
  (Yes, I cut the ground off - I know, I know, shocking) 
It's great BUT now I'm looking at this gadget:   
 
  And I figure with one of these and that, I'm in great shape: 
http://www.amazon.com/Coleman-Cable-6991-Liberator-Extension/dp/B001GLWL0O/ref=pd_sim_hi_5