Apple PowerBook G4
I finally upgraded my wife's long-serving desktop computer in 2005 with a 17" laptop, this PowerBook G4. It was expensive at the time, but it was a great machine. Unlike the Mini I bought at the same time, this computer had enough RAM to last a good long while. Eventually we replaced it with an Intel-based MacBook Air. After the migration was complete I was able to add it to the museum.
As of this writing, I still run Mac OS X on it to play some old games we used to enjoy. (E.g., we can play Myth II cooperatively and combatively between this Mac and my Linux desktop.) And, while our kids were still babies, the big screen served as a late night movie player while one or the other slept soundly between us. I have 10.4 installed even though the last version that supports the PowerPC is 10.5. I just don't see the benefit on this rarely used machine.
Even though it's not Open Source, Mac OS X is a descendent of NeXTSTEP and OpenStep which are a BSD kernel running on top of a Mach microkernel. It's by no means a POSIX user space (SMH, Apple), but it is definitely Unix. This museum is all about running Unix on old computers.