Check out this Tumblr site devoted purely to writing prompts.  Better yet, pull the rss feed directly into your class site.  Below is a screen-cast showing you how to add it to your Moodle course page.

 

 

 

http://content.screencast.com/users/wargoelizabeth/folders/Jing/media/6d2845c9-f4eb-4c59-bcb4-7b7c6f10fdb7/jingswfplayer.swf