zenity-like tool to get multi-element forms with shell scripts?

I like to use zenity to prompt the user for input in in shell scripts, or display information to them - those scripts run on a desktop, and might be minimized or stacked behind other windows, so having a little pop-up is nice. Sometimes I would like to prompt for more than one input field at a time; say, a login and a password. Is there a tool that would do this in a way that is as trivial to add in a random shell script as zenity? The prompt should pop up as a little X window just as zenity does. I have nothing against a more persistent window, but usually don't need it. I do not wish to invert the control flow as is often the case for more "intense" GUI applications (i.e. the control flow should stay as it is in shell scripts, I do *not* wish to make the GUI event loop the main thing, with the shell commands being called from the GUI). If it matters, this is on a Gnome desktop; I am OK with a Gnome-specific solution, but happy with everything else too.