If your product is embeddable on other websites (e.g. live chat, search bar, popup, form etc.), have it link back to your site on free plans, inviting people using it to try it out.
If your product is not embeddable, make users' websites display your company's widget like e.g. Heap and Mixpanel do it. All these users' active pages will be linking back to your site (with a dofollow backlink).