GeoServer is free software and is licensed under the GNU General Public License.:

GeoServer, open geospatial information server
Copyright (C) 2001 - 2011 The Open Planning Project dba OpenPlans

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version (collectively, "GPL").

As an exception to the terms of the GPL, you may copy, modify,
propagate, and distribute a work formed by combining GeoServer with the
Eclipse Libraries, or a work derivative of such a combination, even if
such copying, modification, propagation, or distribution would otherwise
violate the terms of the GPL. Nothing in this exception exempts you from
complying with the GPL in all respects for all of the code used other
than the Eclipse Libraries. You may include this exception and its grant
of permissions when you distribute GeoServer.  Inclusion of this notice
with such a distribution constitutes a grant of such permissions.  If
you do not wish to grant these permissions, remove this paragraph from
your distribution. "GeoServer" means the GeoServer software licensed
under version 2 or any later version of the GPL, or a work based on such
software and licensed under the GPL. "Eclipse Libraries" means Eclipse
Modeling Framework Project and XML Schema Definition software
distributed by the Eclipse Foundation and licensed under the Eclipse
Public License Version 1.0 ("EPL"), or a work based on such software and
licensed under the EPL.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Suite 500, Boston, MA 02110-1335  USA

This product includes software developed by the Apache Software Foundation ( licensed under the Apache License Version 2.0 and Apache License Version 1.1.