The theory of Intelligent Design argues that purely natural forces are not enough to explain the complexity of biology and cosmology, so there must be some sort of intelligence behind it. Problem is, that takes I.D. out of the realm of science. This intelligence is not falsifiable, so it cannot be considered scientific.
However, as a theist, I do believe in intelligent design (it is lower-cased on purpose). I believe that God created the universe (or more particularly, formed it out of pre-existing material), and thus I believe life is intelligently designed. But, that cannot be proven scientifically; it is a matter of faith and non-scientific reasoning.
That being said, I also believe in allowing others to speak for themselves. One of Intelligent Design's strongest advocates and most eloquent spokesmen is Stephen C. Meyer, head of the Discovery Institute's Center for Science and Culture. He is the author of two best-selling books about Intelligent Design, Signature in the Cell and Darwin's Doubt. In this lecture, he outlines the arguments of the latter book and why he thinks Neo-Darwinism is a defective theory. I leave it to the listener to decide if he is successful.