Indeed, volunteering can help a person regarding his or her paid work. Bringing this thread back to the subject of volunteers and technology:
Skills: yes, showing off skills as a volunteer at a nonprofit and building your portfolio can help you in finding paid work. However, don't volunteer for a tech assignment unless you already have the skills needed; nonprofits need tech assignments done by people who know what they are doing, not people who want to try and learn on-the-job. If you are still developing a certain skills, make sure you don't volunteer for something that requires proficiency in that skill; that's not fair to the nonprofit. Volunteering may be a testing ground for you regarding the kind of paid work you want to pursue, but remember that, for the nonprofit you are assisting, there's nothing experimental about your role: the assignments, including tech-related assignments, are real, and they are counting on you to have the skills and experience and commitment needed to finish what you volunteer for.
Looking for a volunteering assignment, particularly a tech-related assignment, is a lot like looking for a paid job: you need to present your credentials and references regarding your work, and you may need to apply several times with organizations and interview more than once before you land a volunteering assignment. Don't assume that walking into a nonprofit and saying, "Ta Da, here I am, put me to work!" is going to get you a volunteering gig. Nonprofits screen volunteers, evaluating their skills and approaches to work just as with paid staff, because tech-related assignments are often very important at the organization, and they need to make absolutely sure they get the right person in that assignment. That means you may get turned down; don't be offended. Just as with a paid assignment, ask what qualities and skills the chosen candidate had; most nonprofits will be honest and tell you why -- and it could lead to your getting chosen for another assignment. You could even ask if you could meet and assist the chosen candidate in the tech assignment, if the nonprofit and candidate feel comfortable with that.
Also, if you are chosen to as a volunteer, be ready to go through the organization's new volunteer orientation; being willing to go through this process shows that you really want to be a part of the organization, and to understand the organization's work and have their mission in mind as you approach your work. Remember that, when it comes to volunteers, organizations aren't just thinking about the work that needs to be done -- they are thinking about how to engage with volunteers, to cultivate them into long-term supporters and advocates of the organization. They usually aren't looking for someone to just do the work; they are looking for a real relationship.
When you finish the assignment, debrief with the staff you worked with at the nonprofit, including other volunteers. How do they feel about your work? Do the feel you have written adequate documentation for a staff member or other volunteer to take over? Will you continue to be available on-call for trouble-shooting?
For nonprofits looking for donated tech help, see Finding a Computer/Network Consultant (paid or volunteer/pro bono).
For people looking for tech-related volunteering opportunities, you have a plethora of places to look, including:
Also contact your local United Way, your local volunteering center, and any local nonprofits that strike your fancy. If you don't see an assignment you are interested in, contact an organization and propose something. Proposing a short-term tech assignment is best, as it will give both you and the nonprofit a "quick win", and could lead to more longer-term, more complex roles.
(FYI, I'm currently helping my local Girl Scouts of the USA council with internal and public communications, mostly as an online volunteer. I landed the gig simply by filling out the online application and showing up at the next monthly meeting at Godfather's Pizza. Best-run meeting I've ever attended -- Girl Scout leaders RAWK).