k, correction: window.open() is the only way to make a new browser window open, not a pop up...my bad (assuming by pop up you mean those cutsey windows that either say something, or have an ok/cancel button, or allow the user to enter stuff).
Of course, to do all that, you'd still be working with javascript. As far as I've found, since asp.net is a server side and not client side (even javascript.NET is server side apparantly), there is no way to create a pop up except with those validation controls I mentioned (and those are limited to just a message, not buttons or anything). What is it you're trying to do?
One way to simulate a dialog is to use a hidden panel, and show it only when needed. But this can be sloppy (it works for our online app, but ours isn't a typical store front or anything)
jack