SGI IRIS Indigo
Inherited one from a coworker. Back in the day, it had been a CAD workstation. He'd been using it as a footrest for some time. I wondered if I could get it to boot. After spending *way* too much money on keyboard, mouse, display and network adapters to convert the proprietary interfaces to the more commonly available PC versions, I powered it up...and was confronted with a demand for a username and password. The SGI systems are quite secure (see below), requiring a ROM password to bypass the default boot process, so I wasn't able to just look at the (SCSI-1) HDD without some effort.
No problem, I thought, I'll just ask the IT guys if they remember what they used back then for a root password. They told me...and it didn't work. However, Google told me that demo/demo might work (it did), so I was able to look through the /etc/passwd file (remember, this is *classic* UNIX). Said file was transferred to a PC using the network connection, and "John the Ripper" was applied to it. By the time I had returned from getting coffee, the password had popped out. And it worked.
Fast forward 6 months later and I get a phone call from the IT guys. Hey, they said, guess what we found while we were cleaning out the safe? An envelope, labeled "UNIX Root Password", which contained a piece of paper, on which was written the password I had recovered.
I managed to find a second SCSI HDD on Ebay, some IRIX OS upgrade CD images on a bittorrent site and created a new boot disk to play on. The system's sitting in my basement. The SGI graphics demos are wonderful, the system boards are impressively heavy and logging in is like taking a trip down memory lane!