Plotbot is a web-based collaborative screenwriting application where you can write a screenplay with as many or as few people as you like. Adopting the wiki approach to screenwriting, each element is editable by any member of a project. You can also comment on, delete or restore any element. Good to use in a safe, closed classroom environment.
This site is really fun, and I think it would work for ALL ages:-) When I use it is when the kids are learning about a plot graph. We will read a short story that has clear elements (exposition, complications, etc), then the kids will have to change the story into their own words, showing me in each cartoon slide, that they understand what those elements are.
This site is a kid-safe search engine that is powered by google. It combines the google safe search with their own restricted elements. It also allows teachers to remove sites if desired. Great for elementary research projects.
faceinhole.com is a fun site with famous pictures of celebrities, movie scenes, music album covers, magazine covers, and more. Each picture has a hole where a face belongs. You can upload a digital photo of yourself and crop it to fit in the hole you chose. You can also post your creation to a blog. These can be done for fun or you can have students create one and tie it into subject matter. For example, they could create a picture of themselves and then use that picture and setting to write a story, using the particular writing elements in your classroom.