Meh. Although some are spiritual, most of the people in my immediate family are atheists or agnostics or both. My mom's probably the most altruistic of the lot of us--she does a lot of volunteering for local causes and once biked a hundred miles (and raised a lot of money, a thousand dollars despite the fact that we were below the poverty line ourselves at the time) for leukemia research--and she doesn't know anyone with leukemia even. It took her months.
Of the rest of us, I almost always give homeless people whatever I can spare, especially when it's cold outside. This winter is particularly bad. I know there are rationals against doing that--that they'll only buy booze or drugs or waste it--but I figure if you're going to give someone a bit of money you shouldn't be judgmental about what they do with it. Regardless of how they got there, all I know is that their lives are much less happy and comfortable than mine, and if they want to spend my five dollars on a bit of liquor that makes them feel better for an hour or two I can't blame them, though I tend to hope they'll buy a real good sandwich instead, if only because it would be more satisfying in the long term.
I can't do much for charitees now, too busy and poor trying to get my degree, but there are a few that I will definitely support when I am in a better position to do so. For now I just ask for Heifer International gifts for christmas and birthday presents.
My brother once gave a guy forty dollars--a lot when you're a poor student without a job--for a bus ticket back home because the guy had just found out his mother was in critical condition from a car crash. He promised to pay my brother back, but never did. We never met him before or saw him again, but he seemed genuinely upset and my brother never hesitated to help. I would have myself but I was completely broke at the time.
All of my siblings have that sort of generous streak. We're quick to pick up a tab or help out a friend or even a stranger, or to volunteer our time or energy or resourses for free. It probably means we'll never be rich, no matter what kinds of jobs we end up getting. You don't get to the top by giving it all away.
So, how do atheists feel about altruism? Well I can't speak for every atheist out there, but of the ones I know it seems we don't feel anything about it at all. We were raised to treat people with love, dignity, and respect. We don't always live up to those ideals, but we do try, and a certain kind of altruism just kind of comes as part of the package.