Teck Loon brought this to my attention: The Shell Economist Prize. I admit that the first thing that leapt to my mind was the prize money. US$20,000 for the top Gold prize. Two Silver runners-up get US$5,000 each, and five Bronze award winners will take home US$3,000 apiece. Not bad for an essay that's less than 2000 words.
Winning essays from previous years are also on the website. Worth a look.