GNU General Public License

This is one of the most popular free and open source licences. In fact, it has become so popular that all the other licences are divided into two categories—licences which are compatible with GPL and those that are not. Two licences are considered compatible with each other if they can be used in a single project without violating the terms of one another. GPL is accepted by both the FSF and the OSI. It is a copyleft licence and often considered one of the strictest licences as far as freedom and openness of a project is considered. It does not allow proprietary projects to even dynamically link to free and open source modules. The only downside is that this licence is often overlooked by developers due to its alleged strictness with regard to software freedom. The latest version is GPL version 3. The Linux kernel and the GCC are examples of software using this licence.