The Affero GPL is a modificated version of the common GPL that explicitly handles the case of software used as a service through internet. It's fairly uncommon

The Affero GPL is a modificated version of the common GPL that explicitly handles the case of software used as a service through internet. It's fairly uncommon, but those projects licensed with this version have specific function built in that allows any user to get a copy of the source code (and you couldn't remove it without breaching the license); and some known names use it (MediaGoblin, MongoDB, Diaspora, Ghostscript, Launchpad, POV-Ray are some examples).