This actually applies year round. We do generally have more time available to volunteer during the summer, though :).
Here are a few ideas:
1. Volunteer at your church. Most churches have a list of opportunities for volunteers (i.e. places they need more help). This could range from assisting the audiovisual department to playing with kids in children's ministry (I have learned a lot about toddlers :)). These are most likely year round positions. Volunteering with VBS would be a temporary position.
2. Become a Global Media Outreach (GMO) online missionary. This is a wonderful opportunity for mature Christians who have a solid foundation in God's Word and hopefully, apologetics as well. I'm not saying that the online missionaries know everything but they should have a firm grasp of correct theology. GMO will put you in a training community when you first join and will teach you how to use their email system and respond to contacts. In addition, there are great resources available to help with answering tough questions such as http://www.gotquestions.org/. I love being able to mentor/disciple/encourage people all over the world...from my home!
3. Search for volunteer opportunities on a website such as Volunteer Match. There are even virtual volunteering opportunities. I was able to assemble a video for a small non-profit in a different state through this website. (Note: I do not necessarily endorse the information posted on this website and/or opportunities you may find on it.)
What about you? Are you interested in volunteering? If you already are volunteering, where/how do you volunteer? Do you have any ideas to add? I'd love to hear them!
Feel free to check out the introduction, first post, and second post of this series.