Created for the GNU project, the GNU General Public License version 2 is the most popular free software license.
Ranked in these QuestionsQuestion Ranking
Pro Guarantees that all changes are given back
Pro Ensures that the software does not become proprietary
Pro Ensures that derivitave works are also protected by copyleft
Ensures that the software and its derivatives remain licensed under the same license to keep the software free and open source.
Con Cannot be linked with other licenses
Software licensed under the GNU GPLv2 is incompatible with all other licenses, with the exception of GPLv3 in most cases. This means that code from a GPLv2 program cannot be combined with code under another license in the same program.
Con Long and complicated
The license is very long and complicated, making it hard to read and comprehend. However, there is a simplified version available here.