"Your intuition knows what it wants to write, so get out of the way" – Ray Bradbury

Writing Prompts
Are suggestions to help you overcome inertia or resistance.

To use them, read the prompt and start writing. Keep your hand moving over the page, or the keyboard. Don't edit, cross out, fix spelling, or worry about punctuation. Just barf whatever's on your mind onto the page without censoring yourself or worrying about it being good, or even making sense.

If you get stuck, try writing "but what I really want to say is..." and complete that sentence. Keep writing for at least five minutes. Gradually increase to ten, fifteen and twenty minute sessions. Try to surprise yourself. Chances are good you'll find revision-worthy gems amongst the rubble.

Adair Lara's prompts and tips.

Cybernetic engine Generates odd prompts.

The First Line
They supply the first line; you finish the story.  If you write something you like, submit it. You could be published in the journal and win $20 for fiction, or $10 for non-fiction.

Use National Geographic's photo of the day as a prompt.

Random prompts in dozens of categories from the Seventh Sanctum. Especially useful for fantasy and gaming.

The Sun Magazine asks readers to send in non-fiction stories based on their prompts for possible publication. Click for topics and guidelines.

Writing Time Writing exercises especially for memoir.

Writer's Digest prompt.

For more info on writing prompts
Writing Down the Bones, by Natalie Goldberg