Distributed SHell. With dsh you can perform a command via ssh on multiple servers. I find it quite handy when doing a lot of the same things on a serverfarm, at least when it's not complicated. For instance, doing a reboot of the whole farm is just 'dsh --all reboot'. I wouldn't recommend it for using it interactively.
With dsh you can make lists of servers, like a list of all your webservers or all servers located in .uk, and only perform the action on that list.
Sort of the same, it spawns multiple ssh sessions and you can input in multiple sessions at the same time. It's extreme usefull when you've got interactive commands, like aptitude dist-upgrade. I found this very usefull when upgrading a serverfarm from etch to lenny.