smart-region

Smart region guess what you want to select by one command. If you call this command multiple times at the same position, it expands selected region (it calls er/expand-region). Else, if you move from the mark and call this command, it select the region rectangular (it call rectangle-mark-mode). Else, if you move from the mark and call this command at same column as mark, it add cursor to each line (it call mc/edit-lines).

This basic concept is from sense-region.