Here are three (free) writing tools that are proven winners with the Kinder - Grade 5 crowd. I like how these tools ignite creativity in even reluctant writers, can be used to reinforce learning in any subject, and how they allow for several different publishing options such as printing or getting a link.
Writing Digitally by Jennifer Rowe BBRSD
StoryMaker on ABCYA.com is the free form option that lets younger students create a picture and then write about it using a lined paper template to reinforce those early writing skills. Kids can either print or save their creations. Potential use: Have kids describe the science or math lesson they just had.
Mystorymaker from the Carnegie Library of Pittsburgh guides kids through the process of creating a short story while also letting them act out the story using characters, actions, moods, settings, and objects. Stories can be saved and accessed via a link. Potential use: Have kids reenact an event they just learned about in history or social studies.
Storybird uses beautiful artwork to inspire creative writing in the form of poetry, long stories, or picture books with captions. Sign in is required but students can use their Google accounts so the process is simple and because Storybird is an educational tool, student won't be spammed later with promotional emails. Potential use: Have kids keep a weekly journal using the chapter book format. Kids can select images that match their emotional profile for the week and write about it (good way to reinforce social/ behavioral awareness too!).