Panopoly add content panel
I just tested installing panopoly distribution and a series of module needed to build same functionality on standard drupal profile installation 

Here are my findings


The functionality you are asking does not seem to achieve with one contrib module instead its a result of lots of modules 
Check this
link. This links lists the feature list or the main modules
list.  
lets take panopoly_pages contrib module, this module
allows user to add content or landing page  
but again this module
depends on several other modules to work in desired way  
Here is
the link for complete list of all required modules.

My findings
Installing all required modules I found it almost become the complete panopoly profile,so in my views it will be better to use panopoly distribution instead of separately installing the modules


How you can add this to existing site ?


https://drupal.org/node/1717728


Check out more on panopoly here.