[DanBC]

Single user mode on an HPUX machine was, I think, the most useful to me.

(http://www.unixhub.com/docs/hpux/hpux_boot.html)

I wish I still had that machine, but I do not.

[RexRollman]

I love playing with OSes and HPUX was one of the ones I didn't get to play with (not to mention Irix and AIX).