Well to everyone can happen to find some extra cool theme that has not widgetized sidebar. Its easy to make your theme widget compatible but some people (like me) prefer to manually edit sidebar and insert needed option to your site.

Here is part of code that inserts Recent posts to your sidebar.

<div class="mycustomclass">
<h3>Latest Guides</h3>
<?php query_posts('showposts=15'); ?>
<?php while (have_posts()) : the_post(); ?>
<li><a href='<?php the_permalink() ?>'><?php the_title(); ?></a></li>
<?php endwhile; ?>

You will add this to your sidebar.php. And add custom styling by defining mycustomclass in your css.