In years past, GPLv2 and GPLv3 were the most popular open source licenses (for projects that had licenses). However, Red Monk co-founder Stephen O'Grady is reporting that more permissive languages are gaining ground at GPL's expense. GPLv2 and v3 are "no longer more popular than most of the other licenses put together," he said. "The three primary permissive license choices (Apache/BSD/MIT) ... collectively are employed by 42 percent. They represent, in fact, three of the five most popular licenses in use today."
O'Grady added that the Apache and MIT licenses have climbed 27 percent, while GPLv2 use has dropped by 24 percent.
Many open source projects have no license at all, a practice which experts do not recommend.