Agree with TandemJeremy about Oxfam – their impact on the world is enormous, and they are pioneers in everything from aid for disasters, initiatives such as Fairtrade, the environment, in addressing global political issues such as trade and debt inequalities, as well as supporting deprived communities throughout the world (including Britain) to help themselves overcome the problems they face.
I would also suggest looking to contribute to a smaller charity local to you which you feel some empathy with. These charities do not have the marketing resources of the large national charities to generate funds, but often do work which has a real benefit for your local community.