Very few SF writing contests catch my eye, but this one from across the pond sounds decent:
"The James White Award is a competition for original short stories of not more than 6,000 words by non-professional writers. The closing date for entries to this year’s competition is midnight (GMT) January 31, 2012. The winner will be announced at Eastercon 2012 during the BSFA Awards ceremony. It is open to entrants from any country but all stories must be in the English language. The prize for the winner of the James White Award is £200 plus publication in Interzone, the UK’s leading science fiction magazine."
A big bonus is that they don't accept entries from professional writers (aka members of SFWA.) More details can be found at the contest page here.