Links

FLTK
Gtk/Gtkmm
Fox
Qt
wxWindows
Tcl/Tk
Rapicorn
Juce, has lots of audio classes

ICC Color Profile
Adobe Postscript
Adobe PDF
Jpeg
Png
Tiff
Midi
SVG
HTML
VRML97
X3D
OpenGL
OpenML
Type1
XML
Unicode
MathML
  (note that the debian package xspecs provides documentation for Xlib and related libraries)
Xlib, might try here too
Icccm
FreeDesktop.org,  has many interesting specifications
OASIS OpenDocument
Yaml, a file format based on indentation.

gegl, the up and coming new infrastructure for the Gimp
libmagick
libxpm
libpng
X11
cups
lcms
freetype
fontconfig
Imlib2
cairo
libart
Enlightenment Foundation Libraries
Antigrain

Autoconf
Autopackage
Zero-Install

Create, an effort to standardize various artsy software things
Fltk based Glasgow infrastructure of the new Cinepaint
Neat up and coming interface equipment, such as multi-contact touch pads. Has do it yourself instructions!
MPX - Multi Pointer X