Exchanging links with sites/customers/colleagues is your only descent option for "free" advertising. You tend to get exactly what you pay for. You might be better off asking what is a particular ad space worth and what is your budget?
Make a video and put it on YouTube. YT receives high traffic, but you must know how to optimize your video so that it gets views and ranks high in the search engines.
Otherwise, you may want to do a lot of social media like Facebook, Google+, etc.. It can bring you lots of traffic as long as you know how to work it.
Generally speaking, i also agree with Mr. Brady. You tend to get what you pay for.
However, depending on your business, Craiglist is usually worth the 15 or 20 minutes per day to keep listings refreshed.
Also, even if you do not plan on paying for advertising on their sites, make sure that your business is listed in the free listing section on YP.com, Superpages.com, Dexknows.com, and any other online directory.
Lastly, for businesses who have the time, but do not have a large marketing budget, take full advantage of Facebook, Twitter, Linked In, and other social media sites.
One can do very well by providing useful, insightful commentary on articles, blog posts, and other matter that is posted quality, industry - specific websites. Its a lot of work, but can be highly rewarding. Good luck!
If it is free, then there must be a catch. Be wary of phishing or virus, Trojans, worms or spyware from such websites. Best to put a dependable anti virus program on your PC unit. It would be better if it is not a free version too.
If you're interested in getting non-targeted traffic, there's plenty of free banner ad space on my website http://commercialplanet.eu .