Though it’s not considered good practice to go around opening new windows on people, it still is something that there are practical uses for. There are legitimate reasons to open new windows, other than trying to open all of your external links in new windows, such as popping a window with a Google Map or Flash game, etc.
I recently came accross an interesting post on WP Cult that discussed opening external links in a new window with jQuery. I adapted the jQuery snippet a little and came up with this:
window.open(this.href, "popupwin", "status=1, toolbar=0, location=1, menubar=0, width=600, height=450, resizeable, scrollbars");
If you add the snippet to your page head, between <script> tags, of course (and after referencing the jQuery library), you gain the ability to selectively make links open in new windows by adding the class of “popup” to the <a> tag.