Om is the most recent addition to the long list of concatenative languages and attempts to be the simplest (and no simpler).
Some defining features of Om:
Om has an extremely minimal syntax. Each program consists of only three syntactic elements: operand, operator, and separator.
Om uses prefix notation rather than postfix, and has an operation queue rather than a data stack. Each operation is a function that takes, as an argument, the remainder of the program and returns a rewrite of the program.
Om has a unique "panmorphic" type system in which there is only one data type exposed in the language (a program), even though the implementation can optimize data representations for operations.
Om is Unicode compliant and implemented using C++, Boost, and ICU.