The GNU General Public License (and its derivative, the Library or “Lesser” GPL) is the single most widely used free-software license. Like the Artistic License, it allows redistribution of modified sources provided the modified files bear “prominent notice”.

The GPL requires that any program containing parts that are under GPL be wholly GPLed. (The exact circumstances that trigger this requirement are not perfectly clear to everybody.)

These extra requirements actually make the GPL more restrictive than any of the other commonly used licenses. (Larry Wall developed the Artistic License to avoid them while serving many of the same objectives.)