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.
There's Mines Advisory Group: https://www.maginternational.org/what-we-do/clear-landmines-clusterbombs/. I'm not sure how effective they are, or how they compare to HALO Trust.