The meaning of life is being alive.
It's all quite simple really. I just formulated this somewhat cogently in the last couple of months (it's January 2000 now), but since I was five I've been thinking about this question.
Always, my answer was that ultimately life is pretty meaningless. That we're all as insignificant as grains of dust in the grand cosmic scheme of things, all created randomly, leading random lives, and that our existence, or lack of, doesn't affect anything. This isn't a negative view---it's an extremely positive and uplifting one (and I've had an extremely positive upbringing). Thinking this viewpoint (assuming it's a relatively correct one) as negative would imply there's something about our place in this Universe that is not satisfactory. Why not accept who we are? I think it behooves us to remove any conditioning that makes us think we're more special than we really are. Only then will be content with who we are and perhaps then able to truly feel alive as a human being.
More importantly, this view opens all possible doors and makes the choices endless. What this means, in a similar vein as Sartrean existentialism, is that whatever meaning to life we give is borne out of our own creation. It is artificial but that's okay, as long as we're content when we go through our chaotic and complex life cycle.
While I believe what I say above is true (and there are many different types of arguments on its behalf), I never could specifically answer the question of what the nature of the meaning that we give to life is. I always felt people frantically searching for meaning and the search was what gave it meaning but yet I felt there was something more than this lack of meaning and the futile search. At first it seemed as though each person's meaning was their own personal thing and finding contentment was what brought meaning to peoples' lives. For example, a mother might find meaning in her life by devoting it to the care of her children; a religious person may find meaning in their life by devoting it to their god (I'm an atheist, for the record); some may find it in the arts, or the sciences, or excelling in some physical activity; and a psychopath may find it by killing other people. No matter how it happens, all of these would/should lead to contentment (and who is one to question or argue what makes one content?).
I think the meaning of life is what you make of it, but what I think this boils down to is doing things that make you realise that you're a living entity, i.e., being alive. For example, I jumped off a plane from 18,000 feet recently. That was an existential experience and what I went through when I experienced that was what made me realise what life is about---I was being alive (or existing). Likewise, when I have an intellectual orgasm solving a problem no one has ever solved before which may change the way the world works, that also makes me feel extremely alive. Or when I ski down a difficult slope successfully or hike the Grand Canyon in less than eight hours. Or having great sex or eating a great meal. Or enjoying a good laugh or a cry. Or marvelling at the beauty of nature and reaching a Zen-like state with it. Or creating this web page. (And so on---in general, doing things you're passionate about makes you realise that you're alive.) That is when I realise I'm alive. I'm being alive. And that is the meaning of life.
If there is confusion about what I'm writing about (I'll refine it as time goes along), think of it this way: what does it mean to be a rock? Or a planet or a star? Or a table? Or a cloud? Or wind? Or a ray of light? That meaning lies in what each object is, the way it is defined and characterised, the way it exists. The meaning of "a ray of light" is just that---being a ray of light. Likewise, the meaning of life is just that---being alive.
Now, of course, one could argue that every moment in our lives is spent being alive. Yet, perhaps because of the way our brains are structured, perhaps because we have some form of self-awareness, we might not necessarily realise it at all points in time. And if we don't realise it for long enough periods, we're not being alive---we're being something else. We lose the passion for life and although we're creatures that are alive, our lives take on a different meaning.
So different people may indeed have an idea of what things to do that make them aware they're alive, but if the things they do don't succeed in this regard, then they may feel their lives lack meaning. Religion, and other institutions, promise this awareness, but my view is that they generally fail (and when they do succeed, more power to them!). I think not being alive, i.e., not doing things that make you aware that you're alive, including allowing yourself to be objectified (you can see why existentialists spent so much thought on this issue), leads to discontentment, forlornness, nausea, and all the other pseudointellectual garbage that existential philosophers talked about.
I guess after writing such an arrogant missive I might as well stop (you can guess the chances of this happening) these pseudointellectual ramblings. I'm sure there are people out there who will take personal affront at my effrontery, but it is my opinion and it is a personal view.
A reader pointed out to me that the meaning of life is simply the biological imperative to transfer our genetic code. This is line with Richard Dawkins' The Selfish Gene, where we're just vessels to propagate our genes and every action we undertaking (including the writing of this missive). I have no philosophical objection against this view (just go to a bar and you'll see evolution in action). I guess my missive above implicitly accepts the genetic framework and then as humans who are sentient and conscious beings, we have somewhat of a "choice" to derive more meaning, but there really isn't any (i.e., it's all chance anyway).