Because why should you pay for something when you can get it for free?
http://www.creativewritingprompts.com/ 329 prompts for creative writing, just like it says in the URL. Sample: "What story can you come up with that has this quick plot: cancer comes back after three years of remission?"
http://www.writersdigest.com/WritingPrompts/ 10 pages of prompts from Writer’s Digest. Sample: "You wake up in jail and have no memory of how you got there. As you pace around the cell, you find five items in your pocket from the night before. As you look at each piece, the night slowly comes back to you. Write about your night, why you have these five items and how you ended up in jail."
http://www.creativity-portal.com/prompts/imagination.prompt.html The Imagination Prompt Generator randomly generates writing prompts for personal non-fiction writing. Sample: "My 10 favorite movies."
http://www.creative-writing-solutions.com/creative-writing-prompts.html A page of creative writing prompts from Creative Writing Solutions. Sample: "A jewel-encrusted box is found in an ancient abandoned temple. Describe the box, what is in the box, and the temple. See where it takes you."
http://www.writingfix.com/right_brain.htm WritingFix’s collection of generators for prompts. Sample: "An intimidating snail was slipping past my ice cream."
http://www.writing.com/main/view_item/item_id/892558 51 prompts collected by a writer. Sample: "Read the following line and create a short story or poem using it as the opening line. And then she died. (Thanks to the Muse’s Alley for providing the line.)."
http://www.mcsweeneys.net/2006/5/4wiencek.html 13 Suggestions from mcsweeneys.net. Sample: "Write a short scene set at a lake, with trees and shit. Throw some birds in there, too.."
http://www.fifteenminutesoffiction.com/prompts.asp Fifteen Minutes of Fiction’s index of weekly writing prompts. Sample: "Write the events or conversations you imagine might happen if two famous people from different centuries were able to meet."