A Stack Based Language based on Joy Language, Forth Language, Lisp Language, and Slate Language. It features the Cons Cell as the building block of code, Higher Order Functions, Call With Current Continuation, Dynamic Typing, and Dynamic Scope.
See www.factorcode.org 
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For a sample, docs.factorcode.org leads to a slick web-based dictionary and documentation browsers for the source of the web server, which supports Web Transactions With Continuations. Super-cool!
Factor is a Concatenative Language.
-- Slava Pestov
For a small sample of Factor code that performs an operation well-known to many Internet users, here's rot13.factor from the "extras" distributed with the Factor system.
! Copyright (C) 2006 Daniel Ehrenberg ! See http://factorcode.org/license.txt for BSD license. USING: kernel math sequences strings io combinators ascii ; IN: rot13
rotate ( ch base -- ch ) [ - 13 + 26 mod ] keep + ;
rot-letter ( ch -- ch )
{ { [ dup letter? ] [ CHAR: a rotate ] } { [ dup LETTER? ] [ CHAR: A rotate ] } { [ t ] [ ] } } cond ;
rot13 ( string -- string ) [ rot-letter ] map ;
rot13-demo ( -- )
"Please enter a string:" print flush readln [ "Your string: " write dup print "Rot13: " write rot13 print ] when* ;
MAIN:
rot13-demo
This illustrates a few properties of Factor code: it looks roughly like Forth, largely composed of colon definitions; control-flow words such as "cond" use data structures composed of code (called quotations) as their arguments; and higher-order functions such as "map" work similarly.
See original on c2.com