I have shown how to show item counts in the Search Refinement Panel previously in this blog post, but you can also choose to show percentages in this field as well.
Below is a image of the standard search refinement results with the Result Type specified to show counts:
Now in order to show percentages instead of count when you set up your ShowCounts attribute in the Filter Category Definition of your Refinement Panel you specify Percentage instead of Count as seen below:
Or easier for us all to copy you simply specify the following to any refinement category you wish:
From there apply the changes, fire off another search and now instead of counts you get a percentage calculation displayed:
Pretty cool out of the box feature that you can use to your particular business problem or just to have some fun
Need more information about the refinement panel, try these links: