OK, the reason the one above didn't work was because of the space in the word javascript.
This is I think because the forum has placed it there, to stop nasty code working on someone elses browser.
However replacing the [ with < etc the one below might work...
[a href="#" onclick="window.open('http://www.google.com','','');"]Google[/a]
<a href="#" onclick="window.open('http://www.google.com','','');">Google</a>
(although this will send the current page to the top of the page, again, not good... but if we're talking general how-to information then there you go)