Ownerless property can be acquired by anyone. Both it and gifts must be acquired by a proper form of acquisition. Rabbinically, however, the gifts of a sick person or a person whose life is in danger are valid by his word alone even if he makes them in secret, and if he does not recover they take effect when he dies. One who declares his property ownerless cannot himself reacquire it for three days; rabbinically such a declaration must be made before three persons. Gifts must not be made in secret, and they are not valid if it is clear that the giver did not intend them -- for example, marriage portions do not become final until the marriage takes place. Pious men avoid accepting gifts, as it says "He who hates gifts shall live".^1,a

Sources:
1. Proverbs 15:27	a. 1:1; 2:1; 3:1; 5:1,4; 6:1,l7ff; 8:2,8,14ff, 24; 9:1; 12:17; see Nedarim 2:14,16-17