Tutorials
A number of people have filled holes in the GTK+ documentation by providing their own tutorials or introductions to specific aspects of GTK+:
The Glib Object system - an introduction to GObject by Mathieu Lacage. (pdf) This tutorial has been merged into the GObject API documentation.
Gtk+ 2.0 Tree View tutorial - a tutorial covering GtkTreeView by Tim-Philipp Müller. (pdf)
Gtk+ 2.0 Text View tutorial - a tutorial covering GtkTextView by Vijay Kumar B.
Building GTK apps for MS Windows on Linux - detailed information about cross-compilation by Allin Cottrell
Articles
GNOMEnclature: The wonders of GLib, Part 1, Part 2 - an introduction to GLib by George Lebl.
The Pango connection: Part 1, Part 2 - an introduction to Pango by Tony Graham.
Localizing GTK+ - an article about input method modules and Pango engines by Theppitak Karoonboonyanan.
Other
Setting Colors in GTK+ - a mini-FAQ by Havoc Pennington.
Windows portability for GNOME software - slides by Tor Lillqvist.