Here's a site that looks good: ChristianVolunteering.org.
Their vision: "To match hundreds of thousands of volunteers with urban ministries and short term urban missions opportunities."
What they do: "ChristianVolunteering.org matches volunteers to volunteer opportunities and helps organizations recruit volunteers through online volunteer matching. In many ways ChristianVolunteering.org is like job matching websites like Monster.com except that we match volunteers to organizations with volunteer opportunities. Individuals can find opportunities as a local volunteer, virtual volunteer (volunteering from home), short term missionary, or intern at organizations across the US and internationally. Organizations can post volunteer opportunities to recruit volunteers online."