Recently, Epinions.com (where I'm a reviewer) ran a writing contest in which people reviewed their favorite online charity sites, and then voted on which charities to donate that month's proceeds to. I wrote about Heifer International, which is still one of my favorites, but Kiva is growing on me too.
What I like about these charities is there's a "teach a man to fish" ethos that goes beyond just slopping a free meal on a tray. Heifer provides farm animals that allow recipients to keep bees, harvest wool, collect eggs, breed animals for meat, and so on. They provide training, and they require recipients to pass on a portion of their gift to neighbors and friends, so they too can become more independent.
Kiva is similar, except they're connecting guilty middle-class white people with impoverished third-world entrepreneurs to make short-term "microloans." Say a baker in the Phillippines needs $500 to buy supplies for her business. A bunch of donors get together and invidually donate small sums ($25 or so), totaling whatever the approved loan amount was. The baker is required to pay back the loan over a period of time, usually in monthly installments, and will provide updates and pictures of how her bakery is going. When the loan is paid back, you'll see your $25 back in your Kiva account, and then you can decide to withdraw it or (ideally) plow it back into another donation for another entrepreneur.
I'm mentioning these sites because we all get hit up for donations constantly, from street hoboes to grocery checkout lines to kids' school fundraisers, and while it's good to act on the impulse to give, you may as well be smart about it and donate wisely. I'm sure there are many other worthy causes out there, though, so if you know of one, leave it in the comments!