couple of times.
Instigation: Creative prompts on the dark side is a book of 500 of them. For all of NaNoWriMo month -- November for those who haven't discovered NaNoWriMo yet! ;-) -- it's half off. (Since it's usually only $3.99, if you miss the special, it's still affordable. :-)
From his website: "This ebook is available for all reading devices (including .pdf) and features 500 "prompts" and includes special help and articles for all you binge writers madly typing away on a novel in the month of November. Offer ends Dec 1st at Midnight, so act quick: Get your copy now for just $1.99."