SSH: It turns out that it's possible to create a live "Master" connection to a specific remote host which then can be used by other ssh command executions to quickly open sub-channels to that host without going through the authentication process every time and without compromising on security.
$ ssh -fMN -o ControlPath=~/.ssh-control-sock remote-host
$ ssh -o ControlPath=~/.ssh-control-sock remote-host date
more about this in ssh(1) and ssh_config(5)
XARGS: In response to some Digg pointer to xjobs, which seems to be similar to xargs only it can run multiple jobs in parallel, someone pointed out to xargs' own -P (--max-procs) argument which does exactly the same. Just have to remember to limit the number of file names passed to each job using -n (--max-args), otherwise all the file names will be passed to a single job.