Widespread licenses

• GNU General Public License (GNU GPL). Based on the idea that all source code for software should be free and everyone should be able to modify anything. However, it is disallowed for proprietary software to build on software released under a GPL license.
• GNU Lesser General Public License (GNU LGPL). Similar to GPL, but permits the use in proprietary software.
• BSD licenses. Makes it relatively easy to distribute software under less restrictive conditions compared with (L)GPL. If a software user wants to redistribute BSD-licensed software, the original copyright and a disclaimer of liability must be included.
• Creative Commons. Allows use in non-commercial projects, as well as redistribution similar to GPL-licensed software.

Example usage of Creative Commons

This work is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License.

Further links

• Comparison of free software licenses at Wikipedia