Assuming you are in the loop, or otherwise know (or know how to get) the $post
object:
function wpse63358_get_post_content() {
global $post;
return $post->post_content;
}
If you want the formatted post content, replace this:
return $post->post_content;
...with this:
return apply_filters( 'the_content', $post->post_content );
To echo the function output:
echo wpse63358_get_post_content();
Edit
Note: if you just want to output post content in the template, you don't actually even have to bother with wrapping it in a function. Simply use:
global $post;
echo apply_filters( 'the_content', $post->post_content );