There has been extended comment here concerning Marx's Labor Theory of Value. This is a subject on which I have written extensively over many years, and I am not going to repeat myself here, but interested readers who are prepared to deal with some serious mathematics are invited to follow the link at the top of this blog to box.net and there to find my paper, "A Critique and Reinterpretation of Marx's Labor Theory of Value." Those somewhat put off by math can read Understanding Marx for a primer.
By the way, Marx's claim that there is a tendency for the rate of profit to fall has been refuted mathematically, first by Okishio and then by Sam Bowles.
If I may summarize twenty years of work in a sentence, Marx was right that Capitalism rests on the exploitation of the working class but wrong that the key to proving this is the distinction between labor and labor power.