The etsy community has been asking for coupon codes forever now, and now they have 'em. It's simple to use. The shop owner just chooses a code with a percentage off and let's people know about it. Then when shoppers check out they just enter the code and the percentage off is automatically calculated and the price automatically adjusted. I'm trying one in my paperwhisperer shop right now. It's HOL2010 for 25% off.
The system is unfortunately kind of limited; for instance, you can't use it only on certain items or items over a certain amount. But it's a start.