[Sodman]

If you're US based, there's Code for America which is exactly that - https://codeforamerica.org . There are many local, typically city-based chapters who meet up regularly and are formed almost entirely of part time volunteers contributing whenever they have time, to whichever project match their skills and interests.

[atlasunshrugged]

Also TechCongress, Presidential Innovation Fellowship, and a few others that have popped up with similar missions/goals. OpenPhilanthropy may do a tech policy fellowship again, if you're in the UK I believe there's a No10 fellowship

[brundolf]

This looks awesome, I'll check it out