xmlroff is a fast, multilingual, multi-platform XSL formatter that transforms XSL-FO documents (or any XML with an appropriate XSL stylesheet) into PDF or PostScript. It aims to excel at DocBook formatting, and it integrates easily with other programs and with scripting languages.
xmlrpc-c is a programming library for writing an XML-RPC server or client in C or C++. XML-RPC is a standard network protocol to allow a client program to make a simple remote procedure call (RPC) type request of a server. It's like SOAP or CORBA, but much simpler. This library speaks the same XML-RPC as similar libraries for lots of other programming languages, with most of the popular extensions. The client library uses either w3c libwww or Curl for HTTP. The server library contains a complete lightweight HTTP (Web) server and also facilities for running with CGI under any Web server.
xmlsd is a library that wraps expat in order to simplify XML use in C programs as well as adding rigid rules to an XML structure. The idea is to have XML structures that can be passed over the network with implicit parameter verification. This in turn enables back and forth chattering between applications.
xmlstack is an ELF binary decoding library that translates all potential executable callstacks for a given binary into XML. This allows for detailed external processing and easier optimization of applications by analyzing the flow of an application. Currently, it only supports x86 ELF binaries.
xmms-crossfade is an output plugin for XMMS. Its main feature is automatic crossfading. It also removes the gaps between tracks, to play whole MP3 albums without any audible interruption. xmms-crossfade supports OSS via a builtin driver, and any other platform by using existing output plugins. As of version 0.3.9, the Audacious media player is also supported.