The Volunteer Center of Santa Cruz County has a great website listing many volunteer opportunities around the community. Check this site for non-profit volunteer opportunities.
VolunteerMatch is a national website, based in the Bay Area, that lets you search by zip code. This site is very easy to use and allows you to search by interest area. Look for the CitySERVE listings!
Idealist.org is also a national website focusing on non-profit jobs, internships and volunteer positions. Although you can find lots of listings when you search for Santa Cruz, this is an especially great site to look at if you are interested in volunteer positions on the East Coast or abroad.
HelpSCC is a great site for finding resources and services here in Santa Cruz County. Although this site is not focused on volunteers, it is a great place to get to know who provides direct services here in Santa Cruz. Almost all of the providers listed use volunteers in some capacity - so call them up and see if they need help!
This is the official site for The Volunteer Initiative Program. This program is the volunteer and internship program for the County of Santa Cruz. VIP is a partnership between the Volunteer Centers of Santa Cruz County and The County of Santa Cruz. Much like CitySERVE, this program has ongoing volunteer programs, internship positions, and one day projects. VIP is also coordinated by the CitySERVE Coordinator.