There have been some attempts at making an IDE for Laszlo, but as far as I know, none of those projects are still maintained, except for the NetBeans OL plugin. It doesn't have a UI designer, though. OpenLaszlo is really a great open source project, but the facts that there was no good IDE for it for a long time, that the only current IDE is a plugin for NetBeans, which you may or may not use, and that there is still no UI designer for OpenLaszlo are all major obstacles for it's widespread adoption.