JGraph is a free, mature, and robust Java diagramming library. It contains all the graph visualization, layout, and interaction functionality you might expect in a graph library, including multiple views, layering, zoom, drag and drop, undo, automatic expanding and collapsing, routing, and layouts. You can create workflow editors, BPMN, CAD tools, network diagrams, database visualization tools, and more. It is fully documented, including examples, and has an active support community.
DirectFB is a thin library that provides developers with hardware graphics acceleration, input device handling and abstraction, an integrated windowing system with support for translucent windows and multiple display layers on top of the Linux framebuffer device. It is a complete hardware abstraction layer with software fallbacks for every graphics operation that is not supported by the underlying hardware.
The Ecere SDK is a cross-platform toolkit for building software applications. It currently runs on Windows, Linux, Mac OS X (through X11), FreeBSD, and the Android OS. It should run on other Unix platforms with minor testing/tweaking. With the Ecere SDK, you can develop applications once and deploy them on all supported platforms alongside a lightweight runtime environment. It introduces eC, an object oriented language derived from and fully compatible with C, compromising neither runtime performance nor ease of use. A built-in 3D engine supporting both Direct3D and OpenGL is fully integrated.
Squeak is an OpenSource and super-portable implementation of a multi-media capable Smalltalk-80 based object-oriented programming environment. It is written entirely in Smalltalk and has a high-performance VM, created by compiling the Smalltalk VM code into efficient and portable C code. It now uses the Self language's Morphic User Interface but still provides the original MVC GUI as well.
Agar is a toolkit for graphical applications. It features a modern GUI system with an unobtrusive API. Agar can use different graphics interfaces, such as SDL and OpenGL, and it has been ported to numerous platforms, including game consoles and embedded systems. Agar is implemented in C, but can be used from other languages.
Vis/Space is a client-server system for visually exploring data structures in 3D virtual space. It allows the placement of various data sources in a three-dimensional environment. Custom search terms can be entered into a query field, or can be navigated to and from an existing object. All objects like images and "text screens" can be moved around. A single click on an image shows the full-resolution version in the foreground. Whole collections of images can be moved. A click to the cube also toggles among several different possible layouts. Data sources include search engines, microblogging, Wikipedia, image/video search, and 4chan.