Linden Lab Official:Second Life Viewer Licensing Program
From Second Life Wiki
The Second Life Viewer source code is available under multiple licenses, which provides businesses and developers the flexibility they need to rapidly and economically achieve their goals. Whichever licensing option you choose, please review our Third Party Viewer Policy. Your rights to modify and redistribute the source code written and provided by Linden Lab is governed under the license of your choice: open source or commercial.
The GNU Lesser General Public License, version 2.1 - those people wanting the freedom to modify and redistribute the Second Life Viewer as open source/free software may do so under the GNU LGPL. Note that the GNU LGPL is a reciprocal license, meaning that if you use portions of the Second Life Viewer LGPL and make changes to those portions, they must also be licensed under the LGPL.
The artwork for the Second Life Viewer is licensed under the Creative Commons Attribution-Share Alike 3.0 License, the complete terms of which are available at the Creative Commons site.
However, please note that all of Linden Lab's trademarks, including but not limited to the Second Life brand name and Second Life Eye-in-Hand logo, are subject to our Trademark Policy.
Note that some components necessary for use with the Second Life viewer are licensed from third parties under different licenses. The license for those components is clearly marked inside the distribution of those components. Some components may require payment of royalties or have other restrictions associated with copying, modification or redistribution. Please consult the license for all components when licensing the software.
For those wishing to incorporate the Second Life Viewer into a proprietary product offering, Linden Lab also offers the Second Life Viewer source code under commercial terms that allow for this. Please call us at (415) 243-9000 for details on the application process.