Where to Register Volunteer Opportunities

General 

Idealist
Allows organizations to list their volunteer opportunities throughout the world.

Volunteer Match
The largest online database of volunteer opportunities, both on site and virtual.

Engage (Points of Light)
Aggregates postings from several major web volunteer opportunitiy registries, but also allows organizations to submit listings directly.

Specific 

Catchafire
Catchafire is dedicated to helping organizations create skills-based volunteer consulting projects and then find professionals to volunteer to complete them.  Note that this service is not free.

Good Work! (Canada)
"We help organizations of all kinds to find environmentally minded staff, managers, consultants, board members, interns and volunteers. GoodWork's mission is to connect those who genuinely care about the environment with the many opportunities — the important work — that needs to be done." 

HealthCare Volunteer
A free portal to connect health professionals (or all kinds) to volunteering opportunities around the world. 

Break Away
A site promoting quality university semester break experiences.

Create the Good
Service started by AARP to reach people over 55 years old, emphasizing flexible-time volunteer opportunities.

Enabling everyone to list, find, coordinate, and measure their impact in the community. 
 
GozAround is a community service connecting the caretakers with those who need care. It is offer volunteer opportunities for organizations. It is not just about financial donations but goes a step further and offers services through organized volunteers from around the world. At GozAround you can post your help and volunteers can earn points through our help programs.
 
Assists in organizing local gatherings about anything, anywhere. This free service has community volunteer topics listed and would be an approach to test if you are seeking people with very special interests. 
 
Brings volunteers together to support local non-profit organizations, with a twist: after each volunteer event -- which typically lasts 3 to 4 hours -- volunteersare invited to gather at a local restaurant or café where they can get to know one another in a relaxed social setting. Has chapters in NY, DC, Chicago, SF, and more.
 
Volunteer America
Opportunities to volunteer through a variety of government agencies