This is one of the coolest photoshop plugins that I have used, GuideGuide. Every graphic designer can benefit form this plugin.
You can download it here: http://www.cameronmcefee.com/guideguide/
To install unzip the file and copy the “guideguide” to your computer. Your platform will dictate where.
Windows: \Program Files\Adobe Photoshop \Plug-Ins\Panels\
Mac: /Applications/Adobe Photoshop /Plug-Ins/Panels/
Thanks to Cameron McEfee