I wrote a while back on the subject of different focus areas for service. That answers the question "what must be achieved to help the world?" but it doesn't answer the question of what communities are involved in those struggles. Thus, it is useful to consider different pathways for service. There are many frameworks in existance to help people think about different pathways, but I haven't seen any that help people think about similar communities. The following is my attempt at such a framework.
Give Time (Direct Service)
General Volunteer
Skill Based Volunteer
Specific Industries
Medicine
Education
Social Work
Etc
Give Money (Philanthropy)
Change Yourself
Be Ethical
Be a Conscious Consumer
Be You (Identity Politics)
Discover How to Create Change
Qualitative and Human Centered (Humanities)
Quantitative and Human Centered (Social Sciences)
Quantitative and Nature Centered (Natural Sciences)
Build a New World
Technology (Engineering)
Organizations, Institutions, and Infrastructure (Social Entrepreneurship)
Lead Good Organizations Well (Business)
Change an Organization
As a Business Leader (Intrapreneurship)
As a Worker (Unions)
As an Outsider (Activist Protest)
Government
Policy Making (Legislators at the Local or National Scale)
Enforcement (Law, Judge, Police)
Execution (Executives at the Local or National Scale)
Change the Culture
Force of Personality (Celebrity)
Force of Truth (Journalism)
Force of Emotion (Art)
Force of Will (Public Intellectual)
Force of Might (Military)
Force of Faith and Devotion (Religion)
Force of the People (Activist Social Movements)