Get some weird and wonderful story starters. Just click on the button to view one of the million starters. There is also a junior version for younger writers at http://www.thestorystarter.com/jr.htm.
This site has a huge number of compelling visual writing prompts with text and questions to get your students thinking. There is something for everyone here!
http://ictmagic.wikispaces.com/English
This is an interesting site that feels like a cross between coding and story writing. Make branching stories where readers choose what happens next.
http://ictmagic.wikispaces.com/English