Why donate anything?
We who earn USD can make a lot of difference for people abroad
We in tech (and non-tech folks too) sometimes have more money than we need
It seems like the best cost-benefit tradeoff to make the world a better place

Your possible objections:
I can’t be sure I’m doing any good.
That’s definitely valid; here I mostly hand it over to GiveWell, who vets their organizations pretty thoroughly. In the end, it is all a bet, but giving to a well-researched organization seems a good enough bet for me.
I don’t have time to research anywhere to donate.
Here again I point to GiveWell, who seems thoughtful enough for me, and I usually just donate to their organizations. They’re interested in doing the most total good for your dollar; it’s sort of utilitarian. If you’re into that sort of thing, I like them; if you’re not, I’d argue that maybe you should be. (related: how personal should your giving be?)
I don’t have money yet, but I will someday. (this likely applies to many of my HCI student friends.)
That’s fair. Especially if you’re a student. Anything you donate now will be dwarfed by your future donations. (with corporate matching, maybe.) In that case, view your current donations primarily as a way to build a habit. Donate some percentage of your annual income. (or, donate some percentage of your income minus some basic level of living expenses. keep that level constant as your income grows.) If you start donating 1% now, that is a not-insignificant chunk, which will become a huge chunk in the future.
I don’t have money yet, and never will.
That’s fine too. Don’t donate, then. There’s no imperative here. For you, it may not be the easiest way to make the world a better place.
I didn’t plan for it in this year’s budget.
That’s fine. Start next year. Pick your target amount, parcel it out week by week. If you make $25k, and you want to donate 1%, that’s $250, which is just $5/week. Put cash in an envelope. (seriously!) Or use whatever money management system works for you; I’m not getting into how you should organize your life.
I don’t think it’s a good way to help the world.
Fine argument. Perhaps you believe that we’d be better off with a free-market utopia or something. I disagree with you, but let’s take that offline. The point is, if you donate money to a well-researched organization, even if you haven’t done The Best, you have still probably done some good, given the state of the world right now.
It doesn’t fit with my current schema of how I manage my money or my life.
This is actually a great point, because it’s really probably the biggest reason not to donate. To which I say, it’s a pretty arbitrary decision; you can make it a part of your life whenever you want. Why not start now?

Where I’m donating this year:
20% to Kiva to help their operating costs. This is because I’ve been lending money through Kiva for years, and don’t usually donate to help them. Kiva (and all microfinance) might not actually help alleviate poverty. Might even promote unhealthy capitalism in the long run, in that Kiva’s organizations are making money off of their clients, and their clients in turn are setting up businesses. Not sure the world needs to be more based on setting up businesses.
On the other hand, microloans do help people smooth out money issues and repayment rates are high, which implies they’re not just setting up a huge cycle of debt.
So I’m kind of ambivalent about this, but like I said, I’ve been lending money through them for years, so my consistency bias in addition to the pluses outweigh the minuses.
80% to GiveDirectly, GiveWell’s top rated charity this year, which is also being matched by Good Ventures.

Side notes:
I was thinking about setting up a “gift matching” thing myself, where I’d match y’all’s gifts up to $x, but it struck me as dishonest for a couple reasons:
1. I was prepared to donate $x anyway, so I’m either lying about the matching, or I’m withholding money that I would otherwise donate
2. We both kind of get too much credit. I get to say “I helped raise $2x”, and you do too, but we each only raised $x.
More on gift matching.

What are your other questions or objections?