CDE is not, in fact, the exclusive product of Sun. Although it's most familiar as the Solaris desktop, it was also shipped as part of AIX, HP-UX, Digital UNIX and, of all things, OpenVMS. Indeed, in the late 1990s and early 2000s, it was present in nearly every commercial Unix with the conspicuous exceptions of IRIX and SCO UNIX. This is because it was actually a joint effort between Hewlett Packard, Sun Microsystems, IBM and DEC. This was an attempt to reduce the divisiveness and gratuitous incompatibility that resulted from the Unix wars, and happened around the same time as the OSF/1 project, but actually accomplished fairly little except to make the various Unices superficially similar while still being very different under the hood. The very fact that CDE ran on platforms as diverse as VMS and AIX should demonstrate that!
But where did it all come from, you ask? Well, the genesis was the HP desktop environment, VUE. VUE looked and felt a lot like CDE, only more bletcherous, and less cohesive. Many of the familiar parts of CDE are direct ports or even renamings of VUE components. CDE's dtpad text editor, for example, is virtually identical to VUE's vuepad. Many components were hacked and improved by IBM's engineers, and were glued together using a modified version of Sun's ToolTalk. The result was the stable and consistent, but clunky, CDE.
Nowadays, CDE is still shipped by HP as part of both HP-UX and OpenVMS, and by IBM as part of AIX. Sun still ships it with Solaris, though since Solaris 10, it's been deprecated in favor of the Java Desktop System. IBM is currently considering whether to include GNOME or KDE instead of CDE in future versions of AIX, but the decision is not yet final. HP has been toying with replacing it on HP-UX for several years, but no decision has been reached yet. OpenVMS is likely to keep it for the foreseeable future, since it's hardly ever used as a workstation OS anymore.
Incidentally, CDE has one feature that's uncommon in other desktop environments (with the notable exception of RISC OS) - you can drag objects from a file manager onto the desktop, leaving only a sort of pointer to the object. In other words, if I drag foo.txt from my home directory and drop it on the desktop, there will be an icon called foo.txt on my desktop now. However, this icon is only a link, or a shortcut to the item in question. Unlike under Mac OS, or Windows, or OS/2, this does not actually create a copy of the file. This is quite familiar to users of RISC OS - it works just like the pinboard - but is a curious, unfamiliar feature for those used to other systems.