Direct Volunteering

However, if you want to connect directly with organizations requesting assistance, this page lists some good starting points.

Go Make A Difference (Go M.A.D.). Go Make A Difference serves as an information clearinghouse for small social projects around the world who are in need of assistance but are unable to communicate their need to prospective volunteers.
They are ideal for people who are traveling to a country and wish to add a volunteer project on to their travels.
See A Volunteer's Experience at Baan Unrak by Anne Cheng for a first-hand experience on a Go M.A.D. project.

Idealist.org. A web-based service of the nonprofit Idealist.org, that matches people directly with organizations seeking assistance. Many projects can be accessed without using an intermediary field organization. Their database is huge, with over 24,000 organizations listed.