I’ve often found that I just want to list posts in the current category.  After a bit searching I came across this:

<?php
foreach(get_the_category() as $category)
{  $cat = $category->cat_ID; }
query_posts
('orderby=date&cat=' . $cat . '&order=DESC');
?>
<?php while (have_posts()) : the_post(); ?>
<li><a href="<?php the_permalink(); ?>">
<?php the_title(''); ?></a></li>
<?php endwhile; ?>

It works a treat.  Just put it within ul tags in your sidebar and style it to suit.