There are lots and lots of opportunities to "give back" using one's technical skills. There is everything from Linux User Groups (LUGs) Install fest assistance; open-source projects of all kinds doing programming, documentation, testing, and other activities; developing software and/or IT infrastructure for non-profits, NGOs and QGOs.
I have been involved in this kind of thing for a number of years. In that time I've had mostly good experiences. There have been a few occasions where I felt compelled to withdraw my support from some organizations when, after working with them for a while, I came to believe that what they purported to be or do was not the case, that in whole or in part their existence was based on tax-avoidance schemes or to create sinecure(s) for the operators of the non-profit rather than provide a real cost effective service to their claimed constituency.
In one case, I came to believe that I was personally at risk if there was an IRS compliance audit because I could be seen to have sufficient information about the organization and operation to be complicit in its bad faith dealing. I left them as quickly as I discovered what I believed to be bad behavior. They had me fooled for quite a while in some cases because they talked a good game, had public and local press support: in effect very good self promotion for unquestionably good causes, were they on the up and up. In all cases they had never undergone a critical outside examination of their financials and operations.
Since that point, I have been very cautious about what kinds of organizations I've been willing to work with. Though I still work with some small non-profits that I've had relationships with for years, I mostly turn away requests from these types of organizations because my experience suggests that a disproportionate number are something other than what they purport to be or that their expense ratios far outweigh the good they purport to do. I now confine my volunteer work to QGOs, such as state chartered volunteer fire departments. The regulatory oversight is better, the paid professionals are better at what they do and the financial controls are very public.
Of course, YMMV. Of course, I could be wrong.