It really depends on the situation for me and changes country to country.
If I get approached with the standard routine for scamming money then I will not give them anything - If however the person approaches me and is genuine then I have no problem with helping them out.
Couple of examples :
*insert elaborate story* can you give me x amount = no
Flip side of this - a guy stopped me outside a liquor store and said, I,m x short for a drink - I gave him the amount.
There used to be a guy that used to be in the neighborhood collecting cans out of the trash - I spotted him a few times per week so I started to keep my cans for him. First time, I called him over and handed him a bag of cans - he was so thankful, as the time progressed I started giving him something to eat, something to drink, clothes and eventually a Christmas card and present :D (I had no idea what his name was, so put to 'My friend' have a great Christmas :D.
If he was desperate he would show up mid week (knowing I wouldn't have many cans for him) - I used to make him something to eat, give him a bottle of water, enough money to buy his next meal and off he would go.
Another story - there used to be a beggar woman that sat in between a well known strip of bars heading towards a club. I spotted her and give her some money - next week she was in the same location so again give her something - next week she recognized me and said hi, are you having a good night. When I actually took the time to sit with her and have a chat she was a really genuine person (the stories she had were worth ever penny I spent on Pizza and tea for her).
If you have no money to spare - spare a few minutes of your time.