Designs in LightI've worked with a number of non-profits in the Bay Area, and strongly believe in their missions. One thing I love about my current work is that it allows me time to volunteer regularly with these groups. These organizations have something in common: giving people a hand up, not a hand out, to break the cycle of poverty and meet people's most basic needs. And my work at them has something in common, too: I feel like I get more out of it than I give, and the more I give the more I get.
There are other non-profits I support, too, but these are the main ones:

I volunteer at my local Habitat for Humanity affiliate a couple times a month, helping build houses and having a lot of fun. I've learned a lot about how to maintain and expand my own house in the process (read about it on my blog, DIY Insanity). But the main reason I do it is because no one should have to live in substandard housing. I've traveled enough to know that there's unfortunately a lot of it not only around the world, but right here at home in the Bay Area. Habitat for Humanity is dedicated to eradicating poverty housing, one house at a time.

Raphael
House -
San Francisco's first family homeless shelter
Every Thursday night, I used help serve dinner to the families and live-in staff at Raphael House. I recently started working with the kids in Raphael House's after school tutoring program, and helping them unwind and have fun on the roof garden after their homework is done. It's simple, very rewarding work, and a joy to watch the kids start being kids again after being there a while. Raphael House is a nurturing, stable environment that gives the families the structure they need to resolve whatever issues led to their homelessness, and the resources to try to help them not become homeless again.
Guide Dogs for the
Blind -
training dogs to aid the visually impaired

Oakland Firefighters
Random Acts of Kindness
- creating a positive difference in the lives of individuals through
Random Acts of Kindness
The Oakland Firefighters Random Acts initiative has been founded by individual firefighters who truly believe they can make a positive difference in other ways besides their emergency response work.
American Red Cross
- whenever
there's a disaster anywhere in the world, the Red Cross is there to
help
I'm involved with the Red Cross in a little more personal way -- I donate blood platelets at my local blood bank. If you're in good health and not too squeamish, this is an easy way to be a hero and safe a life. Think about it -- an hour of your time can save a life.
If you're looking for an opportunity to volunteer, I highly recommend it. Regardless of what you do for a living, regardless of your skills, there is a non-profit group that can use your help. Whether it's an ongoing commitment or a one-day event, there's a rewarding, enriching experience awaiting you. Pick something you believe in, whether it's the protecting the environment, ending poverty, or fighting a disease.
Some resources for finding the right opportunity for you:
If you want happiness for an hour, take a nap. If you want happiness for a day, go fishing. If you want happiness for a year, inherit a fortune. If you want happiness for a lifetime, help somebody. (Chinese proverb)