AGPL
The GNU Affero General Public License (AGPL) is a strong copyleft license in the vein of the GPL, but focusing on server based applications.

Of particular interest is section 13, "Remote Network Interaction; Use with the GNU General Public License."

Notwithstanding any other provision of this License, if you modify the
Program, your modified version must prominently offer all users
interacting with it remotely through a computer network (if your version
supports such interaction) an opportunity to receive the Corresponding
Source of your version by providing access to the Corresponding Source
from a network server at no charge, through some standard or customary
means of facilitating copying of software.  This Corresponding Source
shall include the Corresponding Source for any work covered by version 3
of the GNU General Public License that is incorporated pursuant to the
following paragraph.

Notwithstanding any other provision of this License, you have
permission to link or combine any covered work with a work licensed
under version 3 of the GNU General Public License into a single
combined work, and to convey the resulting work.  The terms of this
License will continue to apply to the part which is the covered work,
but the work with which it is combined will remain governed by version
3 of the GNU General Public License.

which attempts to guarantee the four freedoms to users of a network application.

This has implications for the licensing of projects with AGPL components, especially in languages like Common Lisp where there isn't always a clear distinction between calling an external library and creating a derivative work of that library.

AGPL projects