tmuxssh allows spawning SSH sessions within an existing tmux session, either one SSH session per pane (default), or one SSH session in its own tmux window if -w option is used. If the default one-ssh-per-pane mode is used, all the panes will be created within a new tmux window. This is usually useful if there's a set of ad-hoc commands that have to be executed on a small-ish number of hosts, although I have tested this in cases where number of hosts is >100, just make sure your terminal is big enough to fit all the panes.
Usage:
tmuxssh [OPTION]... [HOSTNAME]...
OPTIONS:
-h Display usage information
-c command Send command to all panes after connecting
-r Connect as root
-s Set pane synchronization to on (keys go to all panes)
-u username Connect as user <username>
-w Open each SSH session in a separate window
By far the most common usecase would be:
$ cat hosts.txt | xargs tmuxssh -rs
This would in turn create a new tmux window in the existing/active tmux session, create a new tmux window, and create a new pane in that new window for each SSH session (connected as root). After all sessions have been created, the panes would be tiled, and tmux synchronize-panes mode would be turned on, so all the keys pressed would go to all panes.