You might not after watching this:
And another video:
I'm always happy to help those in need (see below for the charities I support), but $1 here or there to a guy on the street isn't real help. There are plenty of outlets available to the homeless, hungry, jobless, etc. that will help address the source of their needs, not just the symptoms.
The following are great charities I recommend supporting:
I also urge you to do your due dilligence on charities before donating to them in order to make sure your money is well used. Check out Charity Navigator for ratings on charities.