Turbo Vision is a TUI (Text User Interface) that implements the well known CUA widgets. With TVision you can create an intuitive text mode application, intuitive means it will have CUA like interface.
Universal Related Popup Menus
UniversalIndentGUI is a GUI for several code formatters, beautifiers, and indenters, including GreatCode, AStyle (Artistic Style), GNU Indent, BCPP, and Uncrustify. Its main feature is a live preview to directly see how the selected formatting options affect the source code.
User Dialog Perl Module
User Dialog Perl Module is an OOPerl interface for dialog based applications like dialog, cdialog, whiptail, gdialog and kdialog.
Universal Toolkit is a system written for Perl to provide transparency between toolkits. A program written with Utk can run in GNOME, KDE and HTML (pending Qt bindings) without recoding.
Vector Signal Imaging Processing Library
Vector Signal Imaging Processing Library is an application programming interface (API) defined by an open standard developed by embedded signal and image processing hardware and software vendors, academia, application developers, and government labs.
The vtkgtk project contains the necessary glue for using the VTK toolkit from within a GTK program.
wxWidgets is a cross-platform C++ GUI library, offering classes for all common GUI controls as well as a comprehensive set of helper classes for most common application tasks, ranging from networking to HTML display and image manipulation. wxWidgets uses native widgets on all platforms whenever possible and fills missing gaps on some platforms using generic controls written with wxWidgets itself.
(commercial) X-Designer is a user interface builder for X/Motif, MFC, Java and the Internet.
Xdialog is designed to be a drop in replacement for the "dialog" or "cdialog" programs. It converts any terminal based program into a program with an X-windows interface. The dialogs are easier to see and use while adding even more functionalities (e.g. with the treeview, the file selector, the edit box, the range box, the help button/box).
Xenomai is a real-time development framework cooperating with the Linux kernel, in order to provide a pervasive, interface-agnostic, hard real-time support to user-space applications, seamlessly integrated into the GNU/Linux environment.
XINS is a technology used to define, create and invoke remote APIs. XINS is specification-oriented. When API specifications are written (in XML), XINS will transform them to HTML-based documentation and Java code for both the client- and the server-side. The communication is based on HTTP.
XMPI is an X/Motif based graphical user interface for running, debugging and visualizing MPI programs. Extensive MPI information is extracted from a running application on demand, or from a cummulative log of communication.
xraw is a development kit for creating games, demos, and intros under Linux and derivatives.
xtklib provides a simple, complete API to developers who want to create cross-platform software. In particular, the library provides a full abstraction layer between the APIs and the main services offered by the underlying operating system (eg. threads, processes, GUI, filesystem, etc.) plus a set of generic utilities (eg. strings and data structures).
Yagb is a free GUI builder program for Lesstif. With it you can visually construct your user interface and the program will automatically generate the source code for that GUI. This makes the process of developing a GUI much simpler.
Yaxi is a functional API for OCaml XML data manipulation. It provides implementations of XML 1.0, XPath, and XSLT.
YAZ++ is a C++ programmer's toolkit supporting the development of Z39.50v3 clients and servers. It includes an implementation of the ZOOM C++ binding, a generic YAZ++ client/server API, and a powerful Z39.50 proxy.
Yet Another Eiffel Sockets
Yet Another Eiffel Sockets provides a multi-OS (Win32, Linux, QNX, etc) TCP/IP API layer for Eiffel. SmallEiffel and ISE Eiffel are currently supported.
ZEngine is a collection of classes which are designed in order to make authoring a 2D game easy, allowing the developers to focus on the game rather than complex APIs. ZEngine uses SDL and also uses the addon libraries SDL_ttf, SDL_image, SDL_mixer, and PhysFS.