One place to begin is volunteermatch.org. The nonprofit site is backed by a number of foundations and has corporate advisers from Sun Microsystems, Novell and WorldCom. VolunteerMatch allows potential volunteers to search for opportunities by ZIP code, within a certain radius or within a specified time frame. Most groups can be contacted by e-mail through the site. Hands on Atlanta also offers assignments for teams of friends or co-workers and courses in initiating community action.
Details are on the site.
M.A.J. McKenna writes for the Atlanta Journal-Constitution. J. Foster Co.