Summary of emacs rectangle commands, with default keystrokes:

C-x r k    kill-rectangle
C-x r d   delete-rectangle
C-x r y   yank-rectangle
C-x r o   open-rectangle
C-x r c   clear-rectangle
M-x delete-whitespace-rectangle
C-x r t <text><enter>  replace rectangle contents with text (string-rectangle)
M-x string-insert-rectangle<enter><string><enter>  insert string on each line of the rectangle

from http://www.gnu.org/software/emacs/manual/html_node/emacs/Rectangles.html