The Geospatial Data Abstraction Library (GDAL) is a unifying C/C++ API for accessing raster geospatial data, and currently includes formats like GeoTIFF, Erdas Imagine, Arc/Info Binary, CEOS, DTED, GXF, and SDTS. It is intended to provide efficient access, suitable for use in viewer applications, and also attempts to preserve coordinate systems and metadata. Python, C, and C++ interfaces are available.
GdkRgb is a rewrite of the image rendering subsystem of Gtk+. Advantages over plain Gtk+ 1.0.x include higher speed, very smooth and pretty dithered modes, and support for more displays and visuals. It is currently checked into development versions of Gtk+ (and used in the development tree of the Gimp), but is also packaged separately for application authors who want to maintain Gtk 1.0.x compatibility. The programming interface is quite simple.
Geist is an object-based image editor, designed to be used in conjunction with the GIMP. While the GIMP specializes in pixel editing, geist deals with objects. Geist's specialties are antialiased truetype fonts and a full RGBA colorspace with transparent objects blended quickly in realtime. Supported objects are lines, text (with wordwrapping and font styles), rectangles, images (zoomed or scaled with or without antialiasing), and polygons. Geist saves documents in XML, and can export to all image formats supported by imlib2.
Gepetto provides a framework to animate characters. It lets you record choreographies and build complex moves using elementary actions. Gepetto is fully graphical and offers real-time rendering of the animation, but there is an undelying textual language describing the moves which can be used directly.