If you want to build that killer app, then this is the place to start. take the effort out of coding up all those UI elements and signals. I use this with Builder in Python to link up the code and the UI.
If you need GTK+2 and not 3 then install Glade 3.8 (sudo apt-get install glade-gtk2)
Saves alot of time and frustration with gtk2/perl interface creating. Can't live without it!
Agree with clem. The best and easiest way to make GUI applications for Linux. Recommend it! ;)