Is there a way to hide, stack or collapse menubar icons?
Mac Bartender

I've been struggling with this problem for quite some time as well and I think I've finally found a good solution.

I'm using Mac Bartender which is in free beta at the time of writing. The app has a lot of useful features for cleaning up an overflowing menubar.

For each menubar icon you can choose to:


Display it normally
Completely hide it
Display it in the menubar when Mac Bartender is activated
Display it in the Bartender Bar when Mac Bartender is activated


One thing to note is that it doesn't let you manage the built-in OSX 'menulets' (wifi, sound, spotlight, etc) yet, but that feature is coming according to their FAQ. 

Messy Menubar



Clean Menubar



Bartender Bar



From the official site


  Take control of your Menu Bar
  
  With Bartender you can choose which apps stay in the Menu Bar, are
  displayed in Bartenders Bar or Hidden completely.
  
  Bartenders Bar
  
  Can be displayed and hidden as you want, by clicking on the Bartender
  Icon or via a hot key.
  
  Minimalism
  
  If you want a really clean look and privacy, Bartender can also be
  hidden.
  
  Menu Bar Apps still work the same
  
  Apps still work the same when they are in Bartenders Bar allowing you
  quick access to them when needed, but keep them out of sight when not.
  
  Bartenders Bar can be placed where you want
  
  Bartenders Bar can be moved at any time to any horizontal position
  immediately under the Menu Bar. Or it can open under the Bartender
  Menu Bar Icon.
  
  Show an App in the Menu Bar for 5 seconds when it updates
  
  Set up your Apps to be displayed in the Menu Bar for 5 seconds when
  they update.
  
  Hide Menu from left of Menu Bar to display maximum Menu Bar Apps
  
  Bartender can remove the left hand side Menu temporarily to display
  the maximum possible Menu Bar Apps in the Menu Bar.