Some sites with writing prompts:
Creative writing prompts
Creativity Portal 365 photos
creativity portal writing prompts, more of that kind - and "Daily Meme"
"That's my answer"
10 prompt Thursday
stories without words
Story spinner on-line
Dragon Writing Prompts - has moved: here (try the "Dr Wicked's Write or Die" - and don't cheat! If you don't have anything to write, write down why you don't have anything to write; if you'd need to study more, experience more, read more... then do just that for the next day.)
first 50 words
Adventure Series Photo Writing Prompts
I don't think these are that interesting, but if you are a fantasy/scifi writer, adjust the prompts into your fantasyworld... THEN it becomes interesting :-D
the teachers' corner: march writing prompts
Spring writing prompts