I'd avoid free in this instance. The problem is that the text used in these ads is usually drawn from the vendors description and those words can often be wildly inappropriate - often being info about the afiliate offer. An ad that has its text info about the vendor's conversion rate or payout is not a great way to sell from your page.
We work with a company called CBproads these guys curate the ads themselves and ensure that the text ads are proper 'selly' content. You can set up the ads as widgets in a text widget and best of all you can select the criteria for the ads very closely to suit your site niche.
They offer three levels of service. First is 'free' no restrictions on use but they have a 50/50 split on clicks so that, effectively, they get half of all your sales. Use this level to mke sure the service works and is suitable for your site and business and then migrate to one of their paid services.
Secondly CBProads offer subscription packages for all their services which include what they call shop fronts, banner ads and many kinds of text and image style ads. Again you get 100% revenue
A lifetime package of the text ads with 100% of revenues costs around $20 or less depending upon your choices, with, of course 100% revenue to you.
We tried other services, paid and free, and this is the one we reccomend and use in our site builds.