I generally avoid giving money as I don't know where it ends up and don't want to support substance abuse in any way. I tend to give to charities helping the homeless. When I used to live in London and worked for a huge organisation, I also organised a "buy a lunch" campaign once every few months whereby anyone buying in the staff canteen could either add a set amount of money or duplicate the cost of their meal and all the funds (matched by my organisation) then went towards providing meals and shelter for the homeless....