One example of a free high quality backlink is on the Adobe Education Exchange profile page. It has high domain authority (around 95), though it is a nofollow link.
By creating a profile, one can add backlinks for Facebook, Twitter, as well as a website.
Nofollow links can still be beneficial for SEO, especially from a site with high domain authority and page rank (high DA, PA and PR).
Referral Code Links 
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