GNU General Public License

This license, commonly known as the GPL, has two versions that are actively and widely used in many open source communities:

If you have licensed software you've written under GPL version 2, and you are the original licensor of that software, you may wish to relicense your software under GPL version 3.

Most GPL version 2 software is actually licensed as "GPL version 2 or any later version", or words to similar effect, which is commonly understood as a grant under GPL version 2, section 9 of an "option to follow the terms and conditions . . . of any later version" of the GPL. In this case, even if you are not the original licensor of the GPL version 2 software, you can "follow the terms and conditions" of GPL version 3 instead. This is not commonly done (at least in any explicit way), however.