Where in NYC can I volunteer late on week day nights or on the weekends?

You should hook up with http://www.newyorkcares.org/. They offer loads of low stress, low-commitment volunteer opportunities, and you schedule yourself online, which makes it very easy to work around your own schedule.

Try http://www.onebrick.org

RAINN has http://www.rainn.org/get-involved/volunteer-for-RAINN/ohl-volunteer you can volunteer for and work from home. I realize this doesn't meet your low stress requirements, but I tend to think that all volunteering is stressful on some level, to be honest.

Oh man, you would love volunteering at http://www.thetanknyc.org/. Essentially, you watch some experimental theater or listen to a band or whatever is slated (and you can choose the shows you work) while doling out beer and taking money for the door. Their primary hours are evenings and weekends. You just come an hour earlier and fill up the fridge, sometimes you have to do an ice run to a nearby bodega. After the show, you pick up empties and do trash. Also, you get tips. I think the only time I've felt stressed there is when I had to open new bottles of wine.

What about http://www.housingworks.org/ I spent about six months volunteering in their bookstore. They also have several thrift store locations around the city, if books aren't your thing. I believe there are also volunteer opportunities of the "directly helping needy people one on one" sort (or "directly helping needy people through working on political campaigns and development fundraising", if that's more up your alley), though I'm not sure they're as flexible in terms of time.

The NYC Chapter of http://www.dressforsuccess.org/affiliate.aspx?sisid=50&pageid=1 might be a good place. It is an organization that provides suits to disadvantaged women going to a job interview. Women are referred to DFS by various social service agencies. In addition to the suits and career-appropriate clothing, there are opportunities to help women with resume writing and career development. You'd get to work with women from all different backgrounds and frankly, I've found it incredibly rewarding. As a volunteer, there are actually a bunch of opportunities: fitting clients in their new clothes, working with professional development seminars, or sorting clothes on donation days and working a fundraising event (though these last 2 things won't necessarily involve working with clients directly).