These sites will take flash fiction (usually under 1,000 words, but some even shorter, like Twenty20.) Flash fiction is not necessarily as simple as it may appear.
Short Fiction Break (up to 2,000 words)
Daily Science Fiction
Flash Fiction Online
Foundling Review (up to 2000 words)
Every Day Fiction
Twenty20 (20 words or less)
Brevity (750 words or less)