Artifex v. Hancom
A 2017 US case in which a court accepted that a GNU licence can work as a binding contract, not merely a copyright permission.
Artifex dual-licenses Ghostscript: free under a GNU licence for those who share their source, or for a fee for those who won't. South Korea's Hancom built Ghostscript into its office suite while doing neither – and had, for good measure, stated publicly that it used it under the GNU GPL.
Hancom moved to dismiss on the grounds that nobody had signed anything. Judge Jacqueline Scott Corley in the Northern District of California disagreed: the licence says use constitutes acceptance, and Hancom's own statement was evidence enough. That let Artifex pursue contract damages – essentially the price of a commercial licence – on top of infringement. The parties settled the same year, so the precedential weight is modest, but the reasoning gets cited a lot.