Depends on what you mean by useful for the world. One response could be volunteering for a Wikimedia tech project, another for a charity etc. Another may be to

Depends on what you mean by useful for the world. One response could be volunteering for a Wikimedia tech project, another for a charity etc. Another may be to offer tech support to the old people in your town. Figure out what kind of thing you'd like to help first and then look for projects. Or figure out the type of work you want to do and then look for openings.