i2cssh is a utility that allows for opening multiple terminals to a group of servers and broadcast input into all of the terminals simultaneously. This makes it very easy to operate interactively on a whole group of servers at the same time. i2cssh is a port of the more common Cluster SSH project. While i2cssh depends on the iTerm2 terminal application, there is also the csshx project that provides similar functionality for the default Terminal.app.
When using i2cssh (or split panes in iTerm2 in general) you can toggle broadcasting input to each of the panes within the current tab by using the
Option-Command-i key combination (a warning message will appear that you need to click 'ok' on). Repeating the key combination toggles the broadcast off. Alternatively, right-clicking anywhere in the terminal pane will provide a context menu with the option to "Toggle Broadcast Input" for the given pane. Toggling broadcast across all panes can also be done via the
Shell -> Broadcast Input menu items.