Silex is available under the GPL license, as stated here.
It lets you develop proprietary plugins, but prevents from modifying the software itself and use it without making the altered sources public.
Copyright (c) 2012, Silex Labs
Silex is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License (GPL) as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
To read the license please visit http://www.gnu.org/copyleft/gpl.html
To put in the sources, in each file
* Silex, live web creation
* Copyright (c) Silex Labs
* Silex is available under the GPL license