Load the last post of a category

Hook template_redirect and check if it’s a category archive page, if so get the category ID from query_vars and query_posts for a single post from that category, which by default should be the most recent, then wp_redirect to the post’s permalink.

This code would go in your theme’s functions.php file:

function my_category_redirect() {
    if ( is_category() ) :
        query_posts('cat=". get_query_var("cat') .'&posts_per_page=1');
        wp_redirect( get_permalink() , 301 ); 
        exit; 
    endif;
}
add_action( 'template_redirect', 'my_category_redirect' );