The service works in partnership with many grassroots organizations, international NGOs, local governments, and others in developing countries. Its website, http://www.onlinevolunteering.org/en/vol/index.html, lists tasks (such as writing and editing, translation, research, design, and project development), topics (such as education, youth, food and agriculture, health, and gender), and regions of the world. All you have to do to become a volunteer is to fill out a form online and prepare your curriculum vitae. You will also be asked to specify the tasks/topics/regions you are interested in, as well as the amount of hours per week you can devote to a project. From then on you'll receive volunteer job offers, which you can carry out at home, to your email account. You have to apply for them, like you apply for a regular job, by sending your CV and maybe even demonstrating your skills, such as providing a sample of your writing.
My own limited experience is in the writing and editing field. I did not specify a particular topic or region. My first task was to help prepare material for the newsletter of an African NGO working to promote health and education among the youth of the country. I would not label my experience there a great success as I felt that I was too unfamiliar with the organization and what it was doing, as well as many other factors. Currently, I am working on editing a study carried out in an area of the public sector by a sociologist from another African country. Here I feel more comfortable as editing is different from writing and I'm interacting with only one person.
In addition to the abovementioned problem of insufficient knowledge about a topic/project, another drawback I've noticed while doing this work is dwindling motivation. While you might have other teammates in other parts of the world, you sometimes feel isolated and can lose the will to continue, especially as you'll receive nothing in return and are unlikely to see any results. You are very far from the field where it's actually happening and you have to remind yourself that, nevertheless, you are hopefully helping someone, somewhere, however small your contribution.
If, in spite of what I've said, the idea of online volunteering appeals to you, then at least I've added another helper or two to the UN volunteering pool.