Go Berserk: A book (and website) plus an iPad App to teach children how to build websites. Curriculum is also available for purchase. Check the website for the current cost. $$ Suitable for grades 3-8+
Mozilla Thimble: Thimble makes it ridiculously simple for students to create their own web pages. They can write and edit HTML and CSS right in the browser. Students can instantly preview their work. There are also ready made projects for students to use to introduce them to the basics of HTML. Suitable for grades 4-8+
Mozilla Hackasaurus: Hackasaurus allows students to remix/change/modify their favorite web sties via a visual toolset. It's based on the idea of revealing the structure of web programming to move students from passive consumers of digital media to active producers. Suitable for grades 5+
Create your own unique website with customizable templates.