There's new windows and there's popups. Using target=_blank will open in a new window, except that modern browsers put new windows in new tabs by default. Which sounds like it isn't what you want.

For an actual popup you want window.open(), and be sure to include some specific width and height, otherwise some browsers will still put the new window in a new tab. Darin's example looks good to me.

As for popup blocking, the general approach that browsers take is that popups initiated by user action are allowed (such as clicking), while popups initiated spontaneously through script, such as this, are blocked:

<script type=text/javascript>

    window.open(http://www.google.com/, Google, width=500,height=500);

</script>

However, ad blocking being an escalating war, you can never be sure that a popup will open. If your popup is blocked, the window.open call returns null. So I would modify Daren's example like this:

<a href=http://www.google.com/

    onclick=return !window.open(this.href, 'Google', 'width=500,height=500')

    target=_blank>

If the popup is blocked, onclick returns true, which follows the link they clicked by opening it in a new window or tab. It's a fallback, so at least the content is accessible (if not pretty).