I love the Mac-mini for being such an elegant machine that can be used as a Movie Center, but I have been looking at new applications to make it more useful as a full home appliance.
The beauty of a home appliance is you don't have to bother with learning how to use a computer, all the applications are "dedicated" and can be operated simply with a remote.
Below are a list of things you can do to turn an old or recent mac into a useful home appliance:
Media Center: watching your DVDs, AVI, and downloading all the torrents at the same time thanks to the wonderful PLEX
Plex Media Center for OS X
Virtual Gaming Console: PS2
runningPcsx2 - PCSX2/MAC - Trac
Gaming Central: the 80s
Pure Mac: Emulators - MacMAME
and for the old Apple enthusiasts a wonderful list of emulators, from C64 to the Apple IIe
Apple IIe and more if you can find the ROMs ( I have them all)
Pure Mac: Emulators - Virtual II
Power64
Pure Mac: Emulators - Power 64
Atari800MacX
Pure Mac: Emulators - Atari 800
Colecovision: oh the memory !!!
Pure Mac: Emulators - Mugrat
and finally something more serious if you can have a fixed IP on your network
Mail Server with Web Interface: Running your own eMail ISP service
Open Source Edition: Enterprise Messaging and Collaboration Software by Zimbra