Why Tk?

There are many graphical toolkits around. Why did I pick Tk for this introductory tutorial?

Short answer:

Because Tk is free, easy to use, runs in nearly all platforms and comes automatically with Ruby.

There are several toolkits around. Many of them are much more powerful than Tk? Why did I pick Tk?. Well, here are my reasons:

Tk comes with Ruby automatically. I don't have to take a detour to show you how to install ruby packages. This is an important reason for me.
Tk runs on almost every platform. When you write a program with Ruby/Tk, you can be confident that it'll run on everywhere.
Tk is one of the easier toolkits to learn and use. This is important for an introductory tutorial.
Tk is both free and open source. With Tk, you are not dependent on a software provider who might suddenly decide to stop supporting the product or raise their prices.

There are a few more toolkits that I feel I should mention:

Ruby Cocoa: Bindings for the MacOS X Cocoa. It's probably a good choice if you want to develop for the Mac.
Ruby-GNOME2: Bindings for Gtk+2 and Gnome2. This is my personal favorite. This toolkit is very powerful and easy to use. This is a good choice if you want to develop for Linux or Unix.

Gtk+'s support for other platforms is improving. I hope that one day I can recommend Gtk+ as a multi-platform toolkit.
FXRuby: Bindings for Fox. Like Tk, this is a cross-platform library. Like Tk, it doesn't look native. The Windows version of Ruby comes with FXRuby and Fox. If you plan to develop on Windows, this might be a good choice. A lot of people in the Ruby community like it.