I'm a great fan of writing to triggers or prompts so when I was delighted came across something useful on poet Katy Evans-Bush's blog, Baroque in Hackney. Writing and writers being what they there are times we are teeming with ideas and others when we've got nothing and prompts can push start us when we have a blank space and inspiration fails.
Evans-Bush is a UK poet and poetry teacher and as a writing exercise she directed her class to Bibliomancy Oracle. It's not strictly a prompt site but having played around with it I can see why she suggested it.
Her other link is to 52 Write a Poem a Week . This is, unsurprisingly, a poetry website. I think its prompts and suggested exercises work well for any writer but particularly for speculative fiction writers.
Here are a few other sites and blogs I've found useful but there are many more on the internet.
Writing Prompts - the writer uses them to teach in secondary schools but adults will find them useful too.
Creative Writing Prompts There are over three hundred prompts on this website.
Writers Digest A wide range of prompts on every conceivable subject.
Daily Writing Tips There are some interesting exercises, prompts and links here.
For those of you who like a book to hold in your hands, I highly recommend Everything I Know About Writing by Australian author, John Marsden. It's published by Pan Macmillan Australia and available from the publisher and amazon.com - and, if you prefer it that way, it's also available as an e-book. His fiction is pretty darn good too.