StoryBird: a creative global community of readers, writers, and artists who collaborate to publish work together.
NaNoWriMo for Young Readers: Want to write a digital novel? Check out this site!
Wattpad: a social storytelling platform
Underlined: Read, Write & Connect
Teen Ink: magazine, book series, and website are devoted entirely to writing, art, and photos by teens
One Teen Story: A monthly magazine where each issue features one short story about the teen experience, usually from a known young adult author. Teens can also submit their work for consideration in an annual issue that features a story written by a teen for teens.
Tumblr: a blogging platform for self-expression, but also a site for young literary enthusiasts, bookworms, and those in need of some writing motivation
Also see Tumblr's The Writers' Helper, Writing Prompts, and author John Green's Tumblr for fun and inspiration
Stone Soup: literary magazine 100% written & illustrated by kids
Skipping Stones: a nonprofit magazine for sharing ideas and experiences among youth from different countries and cultures
Kid Pub: website for kids to improve their writing skills by sharing their stories, poems, reviews, and other creative writing with a worldwide audience.
Slightly Odd Writing Prompts: both text & visual
365 Writing Prompts: topics galore for free writes
WritingPrompt.com (13 & below)
Powtoon: Create animated videos and presentations
Padlet: virtual sticky notes for conversations, feedback, ideas, etc
Blogger: create a blog site