I second the Baen's Universe and EscapePod. If you like fantasy at all, Fictionwise has one of Jim Hines short stories. I really enjoy most of his work, long and short.
My other favorite source for short fiction is Black Gate -- it's mostly fantasy. They have a downloadable PDF starting with the last issue...Maybe one before that. Black Gate is my favorite magazine.
Another freebie read you might like is online. Beneath Ceaseless Skies. But it's mostly fantasy too, come to think of it.
Maria
Sage: (multiple formats; Amazon or Smashwords--$1.00)
Catch an Honest Thief (Kindle only format: $1.99; a cozy mystery)