I've had a lot of experience volunteering for non-profit organizations. Granted, it's not the same type of "volunteering" that you mention, but it is a very good path to gain not only coding experience, but leadership skills, business experience, and of course, contacts. On the resume, it is definitely a differentiator. In interviews, I am always asked about my volunteer work.
That being said, there are several pitfalls:
1) The vast majority of non-profits are inherently very conservative and risk adverse due to their unique cash flow situation. You cannot just go into a group and say, "I'll build you X,Y, and Z and it will be fabulous." You must spend time gaining their trust in a volunteer capacity they ask for. If they're advertising on a volunteer site for a programmer, great. You're in. If there is an organization you want to help, but they're not asking for IT help, you may have to spend a long time volunteering for them in whatever role they need, buddy up to the higher ups, then offer advice on how you can streamline things for them.
2) Be careful of the organizations you list on the resume. They might not always help. The homeless, animals, and children are all very good causes that won't offend anyone. Sadly, though, gay and lesbian causes may turn off a born-again HR screener. I'm not saying don't volunteer for controversial causes, but I am saying be careful of what you put on the resume.
3) Be sure you know what you're doing. Even though it is a learning experience for you, it isn't. You are not giving any long term help to a organization by selecting obscure tools and sloppy coding. You will not be there forever. This goes for paid work and non-profit work. You may be hit by a bus, you may have a falling out. However, the product you create will be used for a long, long time. Use best practices and common tools. Mod me down, RoRers, but I recently talked to a non-profit that couldn't find anyone with RoR experience willing to help modify an app that some fly-by-night volunteer developed. They spent months posting on Craigslist and the usual volunteer sites, and eventually had to agree on a complete rewrite in PHP from another volunteer.