The United Nations has a website as part of their Development Programme which links skilled volunteers up with organisations called United Nations Volunteers (UNV).
There are three ways to get started. You can choose a which that matches your skillset, such as translation, consulting, design or project management. You can focus on a specific topic, such as education, health, environment, or health prevention and recovery. Or you can choose to support a specific region, such as Eastern Europe,South/East Asia, or Sub-Saharan Africa.
Once you have chosen your area, you submit an application, and UNV will match you with an organisation in need of your skills. It can sometimes take a little while to hear back, so don’t lose heart if you aren’t placed immediately.
For more information visit the United Nations Volunteers website.