Matthias Clasen has announced the release of GTK +3. This is a major upgrade to GTK+ and offers many new features.

Some of the major changes include, Cairo drawing throughout. GDK no longer wraps the antiquated X11 drawing API. The project has made a clean break and exclusively rely on cairo for all its drawing needs. This has also enabled the team to  remove several other X11-centric concepts such as GCs, colormaps and pixmaps.

Good news for most users is the way GDK was handling input devices. It was a weak point, but GHT +3 brings in XI2 with full support for multiple pointers, keyboards and other gizmos.

GTK +3 offers Easy application support. With the integration of D-Bus support in  GIO, we could finally add a GtkApplication class that handles a lot of the platform integration aspects of writing an application, such as keeping track of open windows, ensuring uniqueness, exporting actions, etc.

You can read complete list of improvemnets hehttp://library.gnome.org/devel/gtk3/3.0/re.

What is GTK +3?
GTK+ (GIMP Toolkit) is a cross-platform widget toolkit for creating graphical user interfaces. It is one of the most popular toolkits for the X Window System, along with FLTK and Qt.

Canonical recently announced they will increse focus on Qt, thus reducing dependency on GTK for their own development.

You can download GTK +3 using below links
ftp://ftp.gtk.org/pub/gtk/3.0/
http://download.gnome.org/sources/gtk+/3.0/