godparents are present at the baptism of the child. They sometimes give their godchildren presents and take them out.

In some cases or perhaps in some places or in the past, I have heard that they would look after the child if the child was orphaned (adopt, maybe, or become the child's guardians)

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Note added at 36 mins (2008-08-14 09:48:08 GMT)
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Google search on '干爹 father':
http://www.google.com/search?q=干爹 father&rls=com.microsoft:e...

I am not for "nominal father" for there is a passage in Leo Tolstoy's "Anna Karenina" reading as follows:

Carried to an extreme, the measures seemed at once to be so absurd that the highest authorities, and public opinion, and intellectual ladies, and the newspapers, all at the same time fell foul of them, expressing their indignation both with the measures and their nominal father, Alexey Alexandrovitch.

Neither am I for "adoptive father" or "foster father" because a 干爹 doesn't necessarily adopt or foster a child.

BTW, it is phonologically significant to have the right tone for 干爹, so that the term wouldn't be mistaken for a "dried father." 台湾大选时，一位民进党的政务官在助选时大声公开说，马英九的父亲把第一声的干爹当成第四声的干爹，这下他的政党败选了，他自己也差点丢掉教职，因为那些道德圣战士在校务会议里投票取消他的聘书，理由：粗口有损师道。因此，要特别小心，千万不要把“干爹”翻译成 dried father, because it is phonologically and linguistically very wrong and very bad.