The Free Software Foundation has published a new open-source software license aimed at developers whose code is used for software-as-a-service (SaaS) applications. The license, called the GNU Affero General Public License, is based on the third version of the GNU GPL.
The GNU GPL mandates that people who distribute applications built from modified source code also release the changes, but it doesn't typically cover SaaS applications. The new Affero GPLv3 includes an additional clause that requires source code to be distributed to users when it's offered as a network service, which is increasingly relevant as companies often use customized open-source software to run their online apps but have no obligation to distribute changes to users. You can read the Free Software Foundation's Affero announcement here.