NSFileHandleExt is an Objective-C extension (Cocoa/GNUstep) for the NSFileHandle class (which handles files/sockets). This extension provides support for much easier network programming for protocols like HTTP, IRC, telnet, etc. (everything that sends messages in lines suffixed by \n).
JIGS (Java Interface for GnuStep) allows Java programmers to use the GNUstep libraries from Java, but it is more than a set of bindings for GNUstep from Java: it takes advantage of the fact that Objective-C and Java are very similar languages to make it possible to use Objective-C classes from Java using exactly the same API (and vice versa). Moreover, JIGS can generate automatically wrappers for your own Objective-C GNUstep libraries.
Étoilé is a GNUstep-based user environment built from the ground up on highly modular and light components with project and document orientation in mind, in order to allow users to create their own workflow by reshaping or recombining provided Services (a.k.a. Applications), Components, etc.
Preferences.app is, functionally, a clone of NeXTstep/OPENSTEP tool of the same name. Just like the original program, it's a simple application for setting preferences for the GNUstep system, with a friendly interface. The program is simple, but powerful, in that people can create new modules, called "bundles", that add new things to the program that its original developers did not think of or even intend, that don't require recompiling the main program.