http://changefol.io is a site that I use to set up micro-donations that's based off my daily spending, e.g., donate $0.05 for every $1 I spend at the gas pump to Sierra Club; $0.10 for every $1 I spend at the grocery store to WWF (World Wild Life Fund).

Personally I love Kiva, "Kiva is a 501(c)(3) non-profit organization that allows people to lend money via the Internet to low-income / underserved entrepreneurs and students in 70 countries." http://kiva.org

The truth and opportunity is that donating to some charities creates disproportionately more good and well being in the world than other charities, so this is a very important question. http://www.givewell.org/ is one charity evaluator that tries to answer this with in-depth analyses. I recommend them!

My wife and I give to http://www.worldvision.org/ (one of the partners listed in the Gates Letter). They've been doing incredible work for over 60 years in both development and aid.