Ok, this is simple: you can visit Oxfam's website and find lots of easy actions that don't require any money or much time and make some kind of a difference. While you're there, sign-up for e-mail campaign updates and do what you can each time something comes up. They are campaigning on really important issues as well: for example anyone familiar with Free Software and their stance on patents and intelectual property will be interested to discover that IP doesn't just cause problems for software, it also causes problems for people in third world countries who can't afford medication due to patents!
Don't just stop at Oxfam either: there are many organisations that need your help such as Amnesty International and Friends of the Earth; both of these have similar "easy" actions which don't require much time or any money. Saying that, if you can afford to give a little money why not!? I'm sure it will be greatly appreciated and will certainly make some kind of a difference.
The point of this post? Do something! Anything! It doesn't matter what as long as you, from time to time, make an effort to do something.