Storyboard That and Make Beliefs Comix. Both services offer writing prompts to use with your students.
Storyboard That offers writing prompts through the teacher dashboard. You have to log-in to your teacher dashboard to find the prompts. A different prompt and or suggested activity is featured daily.
Make Beliefs Comix offers comic strip templates and writing prompts in seven languages. The templates and prompts can be completed online or you can print them out to give to your students. One of the great offerings from Make Beliefs Comix is a free ebook called Something to Write About (link opens a PDF). The free ebook contains dozens of writing prompts. Students can write in the ebook online and print their work. Alternatively, you can print all or part of the book to give to students.
Disclosure: Storyboard That is an advertiser on FreeTech4Teachers.com