Many Unix users may be familiar with ClusterSSH. This tool allows you to open up ssh terminal sessions to many hosts (like when administering a cluster), and then send commands to all of the connected machines simultaneously. csshX is a free, open source tool inspired by ClusterSSH, but designed to work with the OS X Terminal.app (rather than X11's xterm). Once downloaded, the csshX script can be run where it is, or copied into /usr/local/bin/ (or any other directory on your path) for convenience.

The basic command line is:

csshX hostname1 hostname2 hostname3 [...]

This will open three Terminal windows, and ssh into hostname1, hostname2, and hostname3, connecting to one host per window. A fourth (red colored) controller Terminal window will also be opened (this screenshot demonstrates how things look). Any commands typed into the controller window will be sent to all three hosts. You can also send commands directly to only one of the host terminals by selecting its window directly.

For advanced use, menus can be brought up by pressing Control-A. These menus provide you with options such as disabling/enabling input to individual windows, minimizing all the windows, retiling the windows, etc.

[robg adds: This worked as described in testing with my, um, two-machine cluster, and I can see how this would be very useful for those working with a larger number of machines.]