How the hell was OpenOffice missed? Really the only rival I know for MS Office suite.
Also, I just found this one today. Developed by IBM, but available for free, it's Eclipse.. Not 100% sure of all it can do, but from playing with it for half an hour or so, it makes a HELL of a Java IDE. Implemented in GTK2 as well, so it's functional and purrrrdy at the same time. Can't go wrong.
EDIT: I just read a little more on Eclipse. Looks to be big...real big. Does anyone know more about this than I that they can add? It would seem it aims to be a "universal tool platform." Apparently there are projects going on for a C/C++ IDE, Cobol IDE, and many more. Explore the site...it's most worth it.