You may use, at your discretion, either the GPL or the BSD license.

Copies of these licenses have been included in the files
LICENSE.GPL-2 and LICENSE.BSD.

More liberal licensing is available, for example for inclusion in
a commercial product which does not wish to advertise its use of
this software. Users should contact Shevek at <srs@anarres.org> with
specific requirements for licensing or support. Be optimistic. Such
requirements can almost certainly be met.