I co-founded effektiv-spenden.org and legacies-now.org in Germany. Effektiv-spenden.org is a platform that moves donations from Germany and Switzerland to high-impact charities. legacies-now.org supports wealthy philanthropists with a budget of > 1m per year to donate effectively.
In addition to the list Hauke provided: The original founder of vote.org, Debra Cleaver, has left the company some time ago and set up voteamerica.com. We recently interviewed them and were quite impressed by their technology since they have streamlined the voting process so it can be executed by smartphone only (there is the sending of fax involved, that voteamerica.com takes care of). They estimate that it takes them about 3 USD to activate a dormant voter. We ultimately decided to recommend donating to them to a client of ours.