Platform Independent GUI FAQ:
http://www.zeta.org.au/~rosko/pigui.htm
The GUI Toolkit, Framework Page: http://www.geocities.com/SiliconValley/Vista/7184/guitool.html
wxWindows: http://web.ukonline.co.uk/julian.smart/wxwin/
Fresco: http://www.iuk.tu-harburg.de/fresco/
Java (Swing): http://developer.javasoft.com/developer/earlyAccess/jfc.html
GNUStep: http://www.gnustep.org/
ACE: http://www.cs.wustl.edu/~schmidt/ACE.html
Cygnus GNU-Win32: http://www.cygnus.com/misc/gnu-win32/
GTK: http://www.gimp.org/gtk/
GTK--: http://www.cs.tut.fi/~p150650/gtk/gtk--.html
Motif: http://www.camb.opengroup.org/desktop/motif/
Lesstif: http://www.lesstif.org/
Tcl/Tk: http://sunscript.sun.com/
Wine: http://www.winehq.com/
Qt: http://www.troll.no/