Munchkin: Charity: Giving cards to someone who has 5 already
You still give them. But those players must discard extra cards at their charity phase (if they are still the player with the lowest level).

This rule gives a slight advantage to players with the lowest level. Although they mostly get the less usefull cards (gee two halfling and a thief card, I'm really happy now!)

To add some reference to the rules:

The answer is in the rules (the relevant part in bold):


  Your Hand: Cards in your hand are not in play. They don’t help you, but
  they can’t be taken away except by cards that specifically affect “your hand.”
  At the end of your turn, you may have no more than five cards in your hand
  (see Charity, p. 2).


It clearly states, that the hand limit is checked at the end of your turn. So it is no problem if you have 25 cards because of the charity of several players. At the end of your turn you need to level it to 5 (or 6 if you are a dwarf). 

And charity is defined as:


  (3) Charity: If you have more than five cards in your hand, you must play
  enough cards to get you to five or below. If you cannot, or do not want to, you must give the excess cards to the player with the lowest Level. If players are tied for lowest, divide the cards as evenly as possible, but it’s up to you who gets the bigger set(s) of leftovers. If YOU are the lowest or tied for lowest, just discard the excess.