There's a lot of JavaScript for this example http://sethkinast.com/blog/archive/2004/05/21/relexternal/ Or you can use Link Indication Plugin http://sw-guide.de/wordpress/link-indication-plugin/
If you use the regular HTML post editor, and the "add link" edit button, there's an option in the popup to open in same window or open in new window.
Hmm I don't play with CSS too much but you might be able to put the base href target in the css for that content area.
I had a quick look at http://wp-plugins.net/ and couldn't find anything. But you might have better luck.
I wrote a custom one a while ago that might help you out. Create a new php script in your plugin directory and add: /* Plugin Name: Change URLs to open in new window Description: This changes all external URLs to use the blank target (open in new window). It doesn't change local links */ function BlankCallBack ($aMatches) { $lFirstTime = 1; foreach($aMatches as $lURL) { if($lFirstTime == 1) { $lFirstTime = 0; } else { // Done as a callback as we don't want internal links to open in new windows (as that would be annoying) if(strpos($lURL, get_option('siteurl')) == false) { $lURLs .= "<a " . $lURL . " target=\"_blank\">"; } else { $lURLs .= "<a " . $lURL . ">"; } } } return($lURLs); } function st_target_blank( $aText ) { $aText = preg_replace_callback('|<a (.+?)>|i', 'BlankCallBack', $aText); return $aText; } add_filter('the_content', 'st_target_blank', 20); PHP: It's been live on my site for ages so it certainly works but looking at the code now it does look a bit messy .