From Wikipedia.org
How philosophers have addressed the question
Over the millennia, philosophers have had much to say about this question–though philosophers do not fixate on it as much as popular conceptions might lead one to believe. Theories of value–of which there are very many indeed–are not necessarily, but can sometimes be construed as theories of the meaning of life. Great philosophers such as Socrates, Plato, Aristotle, Descartes, Spinoza, and many others had clear views about what sort of life was best (and hence most meaningful). The existentialists addressed themselves to the question head-on. More recently, Robert Nozick discussed the question at great length in his Philosophical Explanations.Using a line of thinking exemplified by Wittgenstein, it could be said that the question is meaningless. This is because ‘meaning of X’ is a term in life usually conveying something regarding ‘the consequences of X’, or ‘significance of X’, or ‘that which should be noted regarding X’, etc..
Things in my life can therefore have meaning (for me, for other people): my life can even be said to have meaning (legacy, achievements, family etc).
But to say that life itself as a whole has meaning is a misuse of language, since any note or significance or consequence would be outwith life and therefore highly dubious in status. The Wittgensteinian line would say therefore that the question is meaningless unless it accepts a realm outwith the realm of life, but this is not usually given.
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http://en.wikipedia.org/wiki/Meaning_of_life
