Once you do one, you'll get calls for more. Often it's nothing more than a few hours on a weekend to set up a website, software package or network. These are generally small concerns who rely on staying small to be effective.
I highly recommend this. I have met some of the most wonderful people.... they have found a purpose in life that transcends its disadvantages. I don't even know how to describe it. Not all are this way, but enough are to bring a smile to my face at the memory.
You will feel like it's the last (insert string of expletives here) thing you want to do when the call comes. Computer work, for free? Just like a job but with none of the advantages, right? When you're done you'll feel differently, even though you will (most likely) be working with substandard hardware in suboptimal conditions around people who understand little of the technology.
I don't really get an ego-boost from it and normally I wouldn't talk about it with people who know me. But I highly recommend it. Among other things, you get a sense of power over all the things you don't like about life, society, computing, etc. A chance to put things right and have people you respect notice.
If you can find an outfit like this, do it:
http://www.volunteerhouston.org/
They are an index of volunteer opportunities, like a Google for volunteering. Once you find your first gig, others will find you.