This is a flag of an activist group called Anonymous. You probably have yet found its motto in a vandalized webpage:
"Knowledge is free. We are Anonymous. We are legion.We do not forgive. We do not forget. Expect us."They are present in initiatives off-line, like the Occupy Wall Street, and on-line, when released a list of names and personal data from people related with virtual pedophilia and cyber-bullying. For its legion of computer hackers, its activities are known as "hacktivism".
This flag has some very curious aspects, in vexillological point of view. The logo of the group recalls the word "Anonymous" and, more surprisingly, has an incredible resemblance with United Nations logo: notice the globe and exact same arrangement of the olive branches.
I couldn't find the symbolism of colors black and green. It can be related to "green anarchism" (anarchism + enviromentalism), an ideology that Anonymous usually defends, although the flag of anarcho-syndicalism defaced with Anonymous logo is also used (see). I have a theory, although I can't prove it: the colors are inspired, conscious or unconsciously, on the old green phosphor monochrome monitors.
Other very curious flag associated with Anonymous group is this:
The anarchist ideas of Anonymous and group's relation with piracy (in contemporary meaning of term) make the Jolly Roger an obvious symbolism (see post about pirate flags). However, they changed the traditional skull by the mask of Guy Fawkes, made famous by V for Vendetta and used by members of the group where anonymity is necessary.
If you know additional information about Anonymous flags or want to make any other kind of commentary, feel free to comment.