A couple of years ago, a Toronto-based musician hijacked Starbucks' Pick of the Week program, where participating stores give away free music download cards.
Preston Clark Edmands was inspired by this and recently did something similar. He's also released a video which shows the process in detail.
If this seems like too much work for you, here's an easier way to do something similar...
Your cards wouldn't have individual download codes, but Overnight Prints has business cards of a similar size with rounded corners and you can print thousands at a time. Most people won't notice that each card has the same code and, although you need to have a code on your card to make it look authentic, you can set up download site so no code is needed get your song.
The main this is to get people to your site and get them listening to your music.