I've been traveling through SE Asia recently, through several countries with extensive landmine problems, and have grown interested in supporting landmine removal and mapping efforts. Right now, the HALO Trust seems like the best option, but I'm interested in other opinions.
Here's another option that detects landmines with rats: https://www.apopo.org/en
Can't comment on cost-effectiveness compared to other similar organisations but it won a Skoll Award for Social Entrepreneurship in 2009 http://skoll.org/organization/apopo/ http://skoll.org/about/skoll-awards/ https://en.m.wikipedia.org/wiki/Skoll_Foundation#The_Skoll_Awards_for_Social_Entrepreneurship