var user_window=window.open('http://www.someplace.com','someplace_window_name','toolbar=no,directories=no,location=no,status=yes,menubar=no,resizable=yes,scrollbars=yes,width=1024,height=768');
user_window.focus();
A user click should initiate this, or it will be blocked by most popup blockers.
This works in all browsers I've had to support including IE6+, FF, Opera, Safari.
The focus bit ensures that the new window is brought to the front.
As an alternative to a popup window, I'd suggest the Dialog plugin for Jquery.
That's what I replaced 90% of my popup windows with.
Your popup becomes a popover (bound within the original window), and this as far as I can tell is never blocked by a popup blocker. The Dialog plugin allows dragable popovers, appearance and disappear effects, and lots of other cool stuff.