There are many different ways to advertise free on the internet. I have personally used many different methods myself and have had success with most of them.
The fastest growing way is social media marketing. To the right person, the use of such tools as Twitter and Facebook can be the biggest source traffic comes from. Promoting whatever company you have, or whatever product your selling on Facebook can make a big difference. The trick though, is to not annoy people with spam. Instead providing incentives that are not too bombarding to sell your products or promote your company.
I have also found blogging to be particularly useful. If you have a particular niche market you are in, or a specific product(s) you are selling, starting a blog can help. Talking about trends in the market, useful tips, etc. can give you the extra traffic you need if you play it right. Networking in blogging is a big part of it. An example would be a tech blog, where you talk about different products, compare them to others, give tips or tricks in tech, and subscribe and comment on other blogs. The more active you are, the more interest you can build.
There are also many different websites who will let you post ads free on them and cycle through them daily. I have found this useful, even though the traffic is not targeted traffic. It still means more sales can be made. If you are on a tight budget, this is definitely a useful tool. There are also targeted niche sites you can post ads for free in exchange for posting their ads on your site.
There is also the e-mail marketing strategy. Sending out newsletters, special offers, etc. to people's emails can bring very good revenue. Though are many people who these types of emails as spam, there are still many people who will take the time to read them.
And two of my favorite types of marketing are video marketing on YouTube and posting and commenting in forums related to your niche product or service. Posting videos on YouTube can be very effective if you are persistent, creative, and honest. Forums play out the same way. And the more active you are in each, the better chance of a higher number of sales.
Also, submitting articles and press releases to sites that could publish them on their site, in their newsletter, or wherever can bring a very high number of people to your site.
These are very general explanations, but they are effective if you use them right. If you, or anyone has questions about any of these please feel free to message me. I would be happy to help in any way I can.