Originally Posted by r1d3r
Well it has a long answer, you can find your answer here
I doubt he wants to be evangelized at this stage.
, your question is too wide. There are lots of free OSes. This forum is about Linux, so we suggest you to use a Linux based OS.
There are lots of so-called distributions. Ubuntu is supposed to be the most friendly for newcomers, but it all depends on the person and his/her needs and tastes.
I suggest you to try a few of them and then decide. You can download some livecds, so you can try them before installing. Some distributions (distros for short) that you could try are Ubuntu, Fedora, Mandriva or SuSE. There are lots more. You can look here to see an almost complete listing: