Café Doom is a writing forum specifically devoted to horror and speculative fiction that can be found here. It's run by Ed Dempster and is one of the friendliest places on the net (although I have to say that I haven't been there as much as I should have done lately, because I've kind of drafted away from horror).
Every week there's a Flash Challenge, where the previous week's winner sets a prompt word and each entrant has to write a piece of 350 words or less using that prompt. Judging is then done by the entrants. The numbers involved can be tiny, but it's none the less competitive for that. Several of my favourite pieces have come out of this.