stack based languages

Hello, i am interested in learning a stack oriented language, so i started learning Forth. (from http://www.amresearch.com/starting_forth/)
Some alternatives i heard of are cat and factor.

What is best to learn from those? And suggest a good tutorial

>>3
Factor has very nice documentation and workspace too... i guess i'll go with factor.
Are stack oriented languages worth it though? Or am i wasting my time with rot swap dup swap over ?