US

If you donate to charities registered in the US you can normally deduct that from your federal income tax. For full details please see http://www.irs.gov/publications/p526/ar02.html#en_US_2011_publink1000229641

This means that it may be best to donate to a US registered charity. Consider someone in the 33% tax bracket. She can donate $150 to a US charity like Deworm the World and then reclaim $50 at the end of the year. Alternatively she could donate $100 to SCI, which is registered in the UK. Both these things cost her the same amount, but the amount of money the charity gets is different. The site Charity Navigator has a simple calculator to make this clear.

Remember that you don't just have to think about federal taxes. There may be special tax arrangements possible in your own state.