WxMaxima
Cross platform GUI for the computer algebra system maxima.
wxWidgets
Library for cross-platform GUI development.
xACT
GUI based front end for unix audio applications. XFlows
XML and XSLT integrated solution. xmlwrapp
Modern C++ XML Parser Wrapper. XPontus XML Editor
A free java based XML Editor.
YaBASIC
Small, fast, cross-platform BASIC interpreter. YamiPod
Yet Another iPod Manager. Zinc
Interactive fiction interpreter.