Depending on your area and schedule, you could try giving time and not money.

As a positive note of this: Some organizations really need volunteers and have easy to meet criteria for work that is not that hard. For instance, my wife started volunteering for a Feline Rescue Association in Baltimore, so on Monday and Thursday Mornings, she drives to the area, cleans up after cats, changes their food/water, gives people adoption papers and references the organization's website, etc.

As a more negative note of this: There are also organizations where they allow volunteers but the labor is substantially heavier, or they actually have a longer application process for Volunteers which may turn people off from volunteering. Also, a lot of the time volunteer places may not have your focus of charity. If you really want to give to Anti-Malaria Charities, there may not be an easy way to do that with time alone.

But it might be something to look into, if you have an urge to do good and very little cash. These kinds of opportunities are going to vary significantly by location.