ViewPDF is an application to view and navigate in PDF documents. It is the successor of ViewIt, which was intended to be a universal preview application for GNUstep. Now, there is Preview.app that plays the role of a preview application, and ViewPDF is focused only on PDF documents. It uses PDFKit to render PDF content.
Netclasses is an asynchronous networking library that works with GNUstep and Mac OS X. Out of the box, it gives a nice, asynchronous, and object-oriented interface for IRC, line- based, and raw TCP/IP connections. Users can easily subclass the raw TCP/IP or line-based interfaces to create asynchronous interfaces for other sorts of connections.
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).