One of my favorite ways to help is with animal photography, there is not a shelter around that can't use the services of a photographer to capture the images of the dogs and cat looking for homes, and it can be your picture that motivates someone to adopt. Many shelters sell calendars as their fundraiser and they can't do it without a photographer.
The hardest was working with "Now I Lay Me Down to Sleep", providing photographic services for families who suffer the loss of a child. About a month after shooting a session for a family that lost their daughter hours after her birth I received a phone call from the dad thanking me for the images and slideshow I had done for them. I finally got the nerve to ask the question "what are you going to do with the slideshow?". His answer drove home the importance of what I had done, "We watched the slideshow everyday for the first week or so, it helped us mourn, and we'll watch the slideshow every year on her birthday to remember and to celebrate her". A year later, on her birthday, I got another thank you phone call.
My latest project is doing portraiture for children with cancer who have lost their hair because of chemo and may not feel as beautiful as they truly are.
I don't tell you these things to impress you, these are things I rarely talk about (and am kinda embarrassed doing so now), but how I feel is not nearly as important as possibly motivating others into action, or to give ideas to those who have been wondering what they can do to help.
One last thought, many photographers who are fortunate enough to be busy are taking a percentage of print sales, session fees, or booked weddings and donating it to organizations that are in the trenches doing great work, like Feed the Children or Operation Smile.
Your camera has the ability to change a life forever, and the life you change may be your own.