Amnesty International work to prevent people with power causing and getting away with the murder, torture, unlawful imprisonment, forced labour and other human rights abuses of innocent people.  Money donated to them goes to researching cases of human rights abuses which may otherwise go ignored, and things like lawyers and campaigns to free people. 

Some info on just one of the things Amnesty have acheived:

The Reverend Samba released from prison in Equatorial Guinea

7 Jun 2008

The Reverend Bienvenido Samba Momesori is released from prison in Evinayong, Equatorial Guinea. He has been held without trial since 2003, and this imprisonment is thought to be politically motivated, due to his known peaceful political opinions and his ethnic origin. 
Prior to his release, Amnesty International had been working on his case for several years and members around the world had taken action on the Reverend Samba’s behalf. 
He later tells Amnesty International “I want to thank you and congratulate you for the work you do and for getting me free. I knew of your work because two years ago I received a letter in prison from a woman in Spain who said she was a member of an NGO. It was a miracle that I received it... Thank you for your solidarity.”

To be honest it will be well easy for me not to drink for a month but there are more important reasons to donate for example one of their current campaigns to help Albert Woodfox who is currently being detained in conditions described by the UN as torture. In fact I'm going to make a seperate donation now.

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