Word Riot first opened up shop in March 2002 and has become one of the best known and most reputable online journals on the interwebs.
According to their submissions page, Word Riot like edgy, experimental writing. They also like publishing the best up-and-coming writers and poets.
In terms of short fiction, Word Riot only ask that submissions should be no less than 1000 words and no greater than 6500. They also request that you don’t explain/tell them about the piece in your cover letter.
Word Riot are also accepting novel extracts, creative non-fiction, poetry, interviews and essays as well as flash fiction. You can see their submissions manager here.