Thanks to Jacqui Cooper for passing on the links to the details of these two competetions.
On the Premises have one with the theme of Darkness and a $220 prize.
The Dragonfly Tea one also has a theme; Discovery. The prize they're offering is £1,500! I could do a lot of discovering with that amount of cash, but sadly can't enter as professional writers are excluded and they define this as anyone who has ever earned a fee from writing.