I would like to transpose columns in matrix like construct like this

1 2        2 1
1 2 -->    2 1
1 2        2 1

I can do this by hand via kill and yank rectangle, but does anybody
know about a package which does this. I searched in the web without
success

From the Emacs Calc manual:

"The v v (calc-reverse-vector) [vec] command reverses a vector end-for-end. Given a matrix, it reverses the order of the rows. (To reverse the columns instead, just use v t v v v t. The same principle can be used to apply other vector commands to the columns of a matrix.) "

You'll have to do some reading to figure out how to get matrices in and out of Calc.