FOX GUI Toolkit
A cross-platform C++ based Toolkit for developing GUIs easily and effectively (Unix/X,Win32). [Open source, LGPL]

ParaGUI
A cross-platform high-level application framework and GUI library based on the Simple DirectMedia Layer. [LGPL]

sjgui
A GUI framework for use in interactive OpenGL applications. The main goal is to provide very easy and fast way to develop portable interactive interfaces, using object-oriented approach. [Open source, BSD License]

Droplets
A rich thin client framework for building software applications that combine the usability and speed of desktop software GUI with the centralized administration and universal accessibility of Web apps.

General Graphics Interface (GGI)
Project that aims to develop a reliable, stable and fast graphics system that works everywhere which will allow any program using GGI to run on any platform requiring at most a recompile.

NCSA Mosaic Common Client Interface
Allows external applications to communicate with running sessions of NCSA Mosaic via TCP/IP. The CCI Application Programmer's Interface (API) supplies programmers with a library of routines to communicate with Mosaic.

LiteClue Widget
A free bubble or balloon help X Widget.

Ygl
Emulates SGI's GL routines under X11. It compiles fine under AIX 3.2, HP-UX 7.0/8.0/9.0, Linux with XFree, SunOS, ConvexOS and others.

Volume 4M: X Toolkit Intrinsics Programming Manual
Orients the new user to window system concepts and provides detailed tutorials for many client programs.

GUI Library Technical Information
The PEG GUI library home page. PEG is a C++ GUI library designed exclusively for real time embedded systems.

libuta
Based on the Simple DirectMedia Layer (SDL). Allows to put together portable graphical applications.

V C++ GUI framework library
Portable, easy-to-use, LGPL. by Object Central, home of the book: Essence of Object-Oriented Programming.

Window to DOS
The projects aims to provide a library of commonly used objects such as a TextBox, Button, Label with scrollable text for DOS. [Freeware]

SansGUI Modeling and Simulation Environment
By ProtoDesign, Inc. Develop and deploy highly interactive scientific and engineering simulation programs without writing one line of graphical user interface (GUI) code.

V
A freeware portable C++ GUI framework for Windows, X, and OS/2.

Volume 5: X Toolkit Intrinsics Reference Manual
Complete programmer's reference for the X Toolkit, providing pages for each of the Xt functions.

Windows++ Code Library
A lightweight C++ application framework for Windows.

GNUStep
An attempt to provide an Object-Oriented application development framework and tool set for use on a wide variety of computer platforms. Based on the original OpenStep specification provided by NeXT, Inc.

xfsft
Patches to provide TrueType font support for X11.

JX Application Framework
JX is a full-featured C++ application framework and GUI class library for the X Window System.

The FOX Dialog Editor
A dialog editor for the FOX GUI library.

TOAD GUI Library
A Simple and Powerful C++ GUI Toolkit for the X Window System.

aedGUI
A cross-plataform, easy-to-use, non-intrusive C++ GUI library that runs on top of SDL. [Open source, LGPL]

LGI (Lightweight GUI Library)
A Lightweight, cross-platform GUI library and application framework. Currently 3 ports exist, Win32, Linux and BeOS. [Open source]

The GUI Toolkit, Framework Page
A comprehensive reference on GUI toolkits, user interface frameworks, software libraries, class libraries,windowing APIs, and related software tools for GUI development.Lists GUI libraries categorized by languages,including C, C++, Java, Tcl, Lisp, and Python. Covers both free software and commercial software.

Vermont Views Plus
A C programming library for creating cross platform user interfaces across Unix, Linux, Posix, VMS, DOS and Windows.

GraphApp
Toolkit for cross-platform GUI programming on Windows, X-Windows, Linux and Macintosh systems. It works with the C, C++ and Python languages, and the source code can be freely downloaded here.

HTML Widget Docs
A list of the X Resources of the HTML Widget.

SmartWin
A light weighted GUI C++ library for developing GUI applications in Windows API.

Imake-Related Software and Documentation
Imake FAQ, software accompanying the O'Reilly "Software Portability with imake" book, notes on use with Motif and X11, imake-related utilities source code.

libUFO: UI For OpenGL
A C++ based core library for GUIs. Uses OpenGL as drawing library and SDL/Glut as windowing toolkit. [Open source, LGPL]

Visual Component Framework
C++ framework created to provide platform GUI framework, features such as Java and Java's Swing, and Borland's Visual Component Library. [Open source, BSD License]

easyGUI
API toolkit for developing GUIs with LCD driver library graphics and graphical display controller drivers. Graphic LCD module application using advanced features like proportional writing, icons, lists and scroll boxes, line drawing.

Realize AB
Tools for building graphical dynamic userinterfaces or DDVT, including: Glg toolkit, a tool for building graphical user interfaces; Visaj, a purely designed for Java GUI's; XDesigner, a tool for building User Interfaces in the X/Motif environment. Demos and source for download.

Bluestone Consulting (BCI)
Provide Unix and Java GUI builder tools such as JClass, JProbe, UIM/X, db-UIM/X, XRT and Platinum.

GLO GUI Library
A platform independent graphical user interface library developed specifically for games.

General Interface
Full windowing and rich client framework written in JavaScript enables full featured application solutions to run in just a standard browser with astoundingly rapid development and deployment ease.