I worked with solar cookers once upon a time and it was difficult to get people to adopt. There's a huge cultural conservatism attached to cooking ad food; it's hard to get people to use different cooking techniques. I suppose that would vary according to the existing cooking culture and scarcity of fuel in any given environment.

This one really works:

http://www.fogquest.org/

Not the single sole solution for everywhere (nothing is) but where conditions are right it's brilliant.

This one:

http://www.biosandfilter.org/biosandfilter/

is another really useful water technology with potential for application in disaster relief or refugee situations. I've built some of these, it's not hard to do and people catch on really fast; they actually get used. That's key... if you track down glorious appropriate tech projects a few years later, a distressingly large number have been discarded.

I admit to a bias toward water supply as an intervention point: it's basic, it's obvious, the payoff is immediate and dramatic, and people get it... if Nelson would have had "want of frigates" stamped on his heart in the event of his demise, anyone who's been in the relief business would have "want of clean water" stamped on theirs. The combination of cheap wind or solar power, batteries, and LED lights is pretty cool, though...