If I pay off a credit card debt for someone using my own credit card does anyone have to pay tax on it?

Probably not. I think you would have two options on how to classify this. You could either treat it as a loan, ie. you loaned money to someone to pay off their credit card. Receiving a loan is not taxable, but there should be a bona fide expectation that you are going to be paid back. Of course, if you are going to charge your friend interest on that loan the interest would be taxable income. The other option would be to treat it as a gift. As long as you are under the gift threshold (in 2008 you can give someone a gift up to $12,000 and there is no tax) there is no tax from anyone. If you are calling it a gift and it was more than $12,000 you may need to file a gift tax return and pay a gift tax. Note that the person GIVING a gift is responsible for paying the gift tax, not the person receiving the gift.