So I've been kicking around this idea for some time now and if done right, it can have more than just a great impact on your business. This one's going to cost you a little, but it's still cheaper that some.
Get some t-shirts printed that feature your company logo and website. I'd go with bright colored shirts, short-sleeve if it's warm out and long sleeve if it isn't.
Now the fun part, "Distribution". You need to do this part carefully as not to seem like you're just a company pushing your brand, but yet a "business that cares about it's community". I'd make up some bags and put together a t-shirt, a bottle of water, an oat bar, etc. Once you have all your t-shirts in a "kit", pack them up and start handing them out, one by one to the folks you see panhandling on the street corners, living under bridges, etc. Chances are pretty high, especially in cold or really hot weather, that they'll add your t-shirt to their wardrobe and wear your shirt for everyone to see. What's better than a billboard? A moving billboard! You're giving instead of buying ad space!
Of course you could just hand out t-shirts to whomever your local audience might be...for instance, most starving college students will do anything for a free t-shirt even if it means jotting their name and email down to get one. Get a crowd going and the frenzy will ensue! With this method, you'll get a good mailing list with a certain demographic, kinda like killing two birds with one stone and still winning the prize!
You can always run a drawing on your website or social media page. Say you'll draw a winner daily or weekly. Winner get's some "company swag" aka a t-shirt. Or take it a step further by creating a store on your site, and the winner get's "store credit" to buy items in your store like a t-shirt.
However you decide to distribute your printed tees, the end goal is to have a living, breathing moving billboard that can be viewed by many and won't cost you a ton in the end.
Here's a couple of places I've used to print t-shirts in the past for several of my businesses and one from whom my family has worked with on various college frat/sorority projects:
Custom Ink - http://www.customink.com
Dad's Specialty Printing - http://www.dadsspecialtyprinting.com
Rush Order Tees - https://www.rushordertees.com