Akashic Books is best know for it's noir anthology series set in various cities. Now they've decided to open submissions for flash fiction to be published at their online site. You can submit stories for either Mondays are Murder which are to feature a distinct location or Thurdaze Flash Fiction which must feature a drug. Stories can be no longer than 750 words, and of course, they must be noir. This is a non-paying market. You can find all the details here.
They've been publishing Mondays are Murder for a while so you can see what they're looking for in a flash piece.