A recent problem found some interesting solutions – I wanted to show a feature on posts of a certain category only.
The feature was the excellent ratings widget from gdragon – the problem was that from their dashboard it was either all or nothing regarding which posts showed the widget.

So I included the following code in the loop (in my case on index.php and single.php) where I wanted the widget to appear

<?php if ( in_category('2') ) { include ('ratings_widget.php'); } ?>

…where ’2′ is the category ID for which posts I want it to show in.
I then created a file called ‘ratings_widget.php’ which included only the placement code for the widget. I then uploaded it with the theme’s php files (next to style.css) via ftp.
The result being that the code was only included in the page when the category was the one I wanted. Easy. Ish.

A similar process can be used to style a particular category of post by using this…

<?php if ( in_category('2') ) { echo ('class="style_for _category2"'); } ?>

(obviously change ‘style_for _category2′ to something better).

Or even include features on particular pages instead of categories with this…

<?php if ( is_home() ) { include ('feature.php'); } ?>

This will give huge flexibilty to layout and feature placement to your blog without having to place the code into every post or creating templates.