These are almost always external projects and not written by SE, so obviously licence issues play some part here. There is a certain functionality that I think would be very useful on some sites and wanted to look around for some existing implementations. I'd like to focus on one or two good candidates and take a closer look, and I'd like to avoid investigating an option that can't be used due to licence issues.
Which of the common open-source licences are acceptable for SE and can be included on SE sites without problems? The major candidates would be GPL, LGPL, AGPL and other copyleft licences where I'm not sure if they would be acceptable or not.