The easiest way to start contributing from the comfort of your home:
UN Volunteers: UNV’s Online Volunteering (OV) service connects development organizations and volunteers over the Internet and supports their effective online collaboration.
Ashoka: Ashoka is the largest network of social entrepreneurs worldwide, with nearly 3,000 Ashoka Fellows in 70 countries putting their system changing ideas into practice on a global scale. Link to virtual volunteering with Ashoka here.
Volunteer Match: This site is a searchable database of 100,000+ non-profits that can connect you to volunteer with a cause that needs you as well as help you discover new volunteer opportunities that matter to you.
Skills for Change: Search engine connecting you to volunteer opportunities depending on your interests and skill set.