www.verifiedvolunteers.com/ Complete background checks for Nonprofit organizations
www.theinterngroup.com/ NGOs and Nonprofits - All Roles As featured on CNN, Forbes & the FT
www.workandvolunteer.com/ Volunteer, Paid work, Gap Year. Make a difference - Apply today!
volunteers.aarp.org/ Register Free With Create The Good To Receive Volunteer Opportunities.
How I did it: (Mostly copied and pasted from my entry from "Volunteer for London Fringe 2008" goal)
I volunteered for the London Fringe Festival last year, after finding out about it from friends who volunteer at the same theatre I did in the summer. They were volunteering at Fringe as well and told me where to go to sign up. I checked out the website, and just went to the Fringe headquarters and asked if they needed any help!
Lessons & tips: Just ask! Get your courage up, email/call/visit the headquarters, and ask if they need help. Most places seem to be great with this! And non-profit organizations generally need all the help they can get!