North American Mission Handbook: U.S. and Canadian Protestant Ministries Overseas edited by Peggy Newell
Mission Handbook contains information on nearly one thousand agencies working overseas. Aside from the agency listings with contact information, types of mission activities, areas of service, personnel statistics, and more, the agencies are also listed by church tradition, by ministry activities, and by country of activity.
Ask a Missionary has a series of questions and answers on topics related to support choosing a sending agency.
The Mission Finder website contains a search function to help prospective missionaries find an organization or a trip that matches their interests and giftings.
A British organization, OSCAR has a comprehensive list of missions organizations with a UK base of representation.
The Mission App allows those who are interested in serving in long-term missions to fill out an application and be matched with organizations which fit your skills and calling. Applicants' information is then passed on to those agencies, which will be in touch with the applicants directly.