I want to mention the excellent Writers' Village website, as many short story writers may not be aware of it, and its quarterly short story competition. Run by John Yeoman, author and a creative writing lecturer, the next short story competition deadline is 30th June 2010.
As well as first, second and third prizes, there are also ten generous 'shortlisted winners' prizes of £10. All winning entries are published online. I was lucky enough to be one of the shortlisted winners in the 2010 spring competition, and all winning entries from this and the previous competition can now be read on the website.
In addition to the competitions, there is the opportunity to receive regular email tips on what makes a good short story. Full details can be found here.