The following code will display a unique RSS feed on each “category archive” page within your WordPress blog or site. The feed will contain only the posts within that category. If you have any questions about the code, or if you just like it a lot, leave a comment to let me know.

<?php /* If this is a category archive */ if (is_category()) { ?>
    <span>
    Subscribe to posts in <a href="/wp-rss2.php?cat=<?php echo get_query_var('cat'); ?>"><?php single_cat_title(); ?></a> via RSS
    </span>
<?php } ?>

To use it in your sidebar, just make sure your wrap the entire chunk of code in <li class="widget"></li> tags so that it is recognized as an element of the sidebar <ul>. To add a title in the sidebar, wrap it in <h3 class="widget-title">Recent Posts</h3>, where “Recent Posts” is your desired title.

<?php query_posts('showposts=5'); ?>
<ul>
    <?php while (have_posts()) : the_post(); ?>
        <li><a href="<?php the_permalink() ?>"><?php the_title(); ?></a></li>
    <?php endwhile;?>
</ul>

Questions? Drop ‘em in the comments

<li class="widget">
  <h3 class="widget-title">Categories</h3>
  <ul>
    <?php wp_list_cats('sort_column=name&optioncount=1&exclude=10'); ?>
  </ul>
</li>