I have worked across a plethora of activist organisations over the years, from People for Nuclear Disarmament, to organisations supporting independance of East Timor and Pacific colonies, as well as feminist and overseas aid organisations.
Activism in NGO's is a great way to meet like minded people as well as help others, by liaising with them to lobby, support and campaign about their concerns.
Some NGO's are big enough to accomodate volunteers, if you are not into contributing via meetings and directly influencing policies, action and endeavours. These include organisations like the Red Cross or Cresent, Oxfam, Refugee Support Centres, animal welfare organisations such as the RSPCA, Cat Protection Society, Lost Dogs Home and environmental organisations, such as Landcare, or Clean Up Australia.
Being involved in community based organisations is a great means of boosting your contribution to the world beyond your regular employment. It is a simple act of citizenship. If you are active in a few NGOs, you will likely find you soon develop networks and friendships all over the country and sometimes around the world!