Here are a few suggestions to get you started. If you know of any other sites, please let me know and I'll post them here.
Available Tutors (on-line)
Doug Shy - Doug is a former physics and mathematics grad student. He has over 15 years experience as a tutor and is currently a math department chair at a private high school in Los Angeles. He is willing to answer questions on-line, by email, or in person.
Posting or Finding Tutor Ads
FindaTutor.org - A wonderful non-profit site aimed at connecting students with tutors over the Internet. A great place to start your search.
CollegeClub - A popular site among university and college students. There is a "Tutor Finder" section here, but I haven't seen any volunteer tutors listed here yet. I think you have to create an account at www.collegeclub.com to get access to the listings.
Online Tutors and Tutorials
Big Chalk - Also called "Homework Central", this site has tutorials on all kinds of topics at the high-school and post-secondary levels.
Tutorial-Find - Lots of tutorials for school and non-school related stuff. Many of the tutorials are computer/internet related, but you may be able to find some useful stuff here.
Math Tutorials - Links to a bunch of math tutorial sites and a few people who will help you with math questions. Very useful.
Other Sources of Online Help
Online Tutoring World - The one and only FREE online tutoring resource for esl teachers.
Struzzin Company - This site has radio shows offering help with questions in Math (Algebra, Geometry, Trigonometry, etc). Check out Terri Lynn Easter's show Math Flare and other great shows. You can send in questions by email and they will be answered during the radio show. You can also look up archived questions and answers from the past few weeks. Note: I think you need a Shockwave Flash player to use this site.
Products and Services - This site has links to all kinds of education-related pages. Plenty of information about tutoring as well as educational tool suppliers, home schooling tips, and online tutorials.
Any other links that volunteer tutors should know about? Tell me about them!