The IDE4Laszlo is on old IBM project, which has not been maintained for a long time (since 2005/2006). Laszlo had been working on an Eclipse based plugin in 2008, which was unfortunately never released.
Spket works, but compared to IDEs like Flash Builder the functionality is relatively limited, since the IDE is not capable of scanning your application files to recognize new classes and methods.
An option is to use an XML editor with XSD support. Sebastian Wagner of OpenMeetings has created a build script which generates an XSD schema file from the OpenLaszlo classes and the custom classes in your application. The source code can be found here:
http://code.google.com/p/openlaszlo-schemagenerator/