Search Agency Slingshot SEO has compiled this very interesting and useful report on click rates on each position in the organic search results, reported in the linked article at Search Engine Watch.  I'll include the graph here:

*First name:  
Last name:  
Email:  
*Comment:  
**Enter the security code shown below  
 

The purpose of this picture is to protect the owner of this site against spam bots.

 
 
 
  * Indicates a required field.
  ** This is to prevent unauthorized automated scripts.