I believe in Karma in a certain sense. The whole idea that whatever good you do comes back to you and vice versa is one that can often be true for people. However, that truth is not determined by some supreme being who has instilled karma into the world, but derives out of actions and behaviors.
For example, it is more likely that a good natured, caring, humble person who goes out of her way day after day to help people will find herself bestowed with a similar good fortune, whether it be from others who want to reward her or perhaps a benevolent feeling of accomplishment and self-worth for the good she has done in the world. The same can be said about bad deeds.