ShortTermMissions.com helps people quickly connect with short-term mission trips that fit their gifts, calling, and availability, listing hundreds of mission trip provided by many agencies. A great source for articles and questions about short-term missions.
Mission Finder lists hundreds of short term, medical, domestic, and international mission trips, categorized by location and type of ministry. Lots of links, short descriptions.
Mission Next is designed for adults of all ages. The same ministry provides Mission:TEACH, placing teachers in international Christian schools.
Christian Connections for International Health and MedicalMissions.com are designed for medical personnel and those interested in medical work.
ChristianJobs.com is not specifically focused on missions opportunities, but lists options for employment at ministry organizations, nonprofits, and Christian businesses.