Simpler Web Design with HTML5 Prototyping Services
Web is becoming a second desktop now. And the requirements for web apps goes the same as for desktop apps. Especially for the UI part. It's a bad idea to start developing UI in Photoshop. You can't test application behavior, responsiveness, grids, on static image. Same goes for oldschool paper+pen method. You just don't aware of poroportions and sizes of your components, so drawed wireframes will look completely different when they come to HTML+CSS.
Modern way or prototyping stands for opening text editor and writing some HTML code with Twitter Bootstrap included. But actually why would you do this if there are services that does just the same in WYSIWYG way? It is much easier to try different elements, positions, just by moving them through pages, without editing HTML or CSS. So why not to use this tools. You may stick to well known Bootstrap, use Foundation or custom framework.
Let's list them all. Choose the tool that fit your needs better.