I found a great website: Volunteermatch.org. Volunteer Match is just what it sounds like - they match up organizations in need of volunteers with people looking for ways to help. According to their site, they currently have 68,837 participating organizations, 59,077 volunteer opportunities, and they've made 4,266,106 referrals since 1998.
Through this site I found an opportunity to volunteer with the American Cancer Society. I went to training yesterday and soon I'll be driving cancer patients to and/or from their doctors' appointments. Before I read about this on the site, I had no idea that there was a need for this. It makes sense, though, since so many people live alone or with people who can't drive them to their appointments. I'll let you know how it goes after I've gotten started. I'm really looking forward to it.
Let me know if you find a volunteer opportunity that works for you.