Velositey is surely one of the best web design extensions around for Photoshop. Version 2 of this extension offers a bunch of excellent functionality designed to save you time when prototyping and creating layouts for your websites.
This extension is really quick and easy to use, is bootstrap compatible, and has a pre-generated 1170px grid system (you can create a new document with a single click). There are a whole host of elements that can be deployed including multiple headers, sliders, footers, modules and content areas. A mapicon and favicon generator is also included.
Velositey 2 is compatible with Photoshop CS6, CC & CC 2014, and is completely free.
Check out their website to find out more.