Reflex Fiction does a quarterly writing contest with a huge prize, specifically for fiction between 180 and 360 words. They longlist entries, then do a countdown where every day they post a piece of flash fiction that was entered (you can opt out, but they'll publish a marketing link) until they get to the winners. You'll also be eligible to have your work placed in their anthology, which does not pay dollars, but you get a free copy. They have a "suggested" entry fee.