<?php if (current_user_can("manage_options")) : ?>
       <a href="<?php echo bloginfo("siteurl") ?>/wp-admin/">Admin</a>
<?php endif; ?>

If a user is logged in and they are an Admin of the site (not just an subscriber or author), then display a link to get them to the WordPress Admin area. Otherwise, display nothing.

More specific than the regular log in / log out function, which will show for everyone:

<?php wp_loginout(); ?>