The Volunteer Union

Links to Organizations that coordinate volunteers...
[this page is looking for comments and volunteers!]

An environmental effort that we heard of once to encourage cross-organizational volunteering:
http://togethergreen.org/Volunteer/CommunityPassport.aspx

One Brick
Fun & social volunteer activities. Lots of event-based one-time opportunities.

olunteermatch
"Where Volunteering Begins"

http://www.idealist.org/
Good for people thinking about nonprofit careers after volunteering, or for connecting with other volunteers (I think)

http://www.networkforgood.org/
Volunteer or donate. Lots of links.

http://www.voa.org/
Human services focus.

http://www.servenet.org/
Youth focus.

http://www.volunteerabroad.com/search.cfm
International focus.

http://www.thevolunteercenter.net/index.html
San Francisco area, includes the Board Match for more serious volunteers.