Is the Sun Nuclear DICOM Listener still available for download?
No, the software is not available for downloading. I have checked the homepage and it appears the DICOM Listener is bundled with MapCHECK® 2. If you acquire MapCHECK 2, then the DICOM Listener will also be available for starting after the MapCHECK has been installed on your computer. But this download leads to another "issue". The MapCHECK 2 application is not available for public downloads which means you need to reach the developers in order to obtain a possible download.
This can be done through the official website: http://www.sunnuclear.com/medPhys/patientqa/mapcheck2/mapcheck2.asp