I'll think of more later...
A cheap hard drive (suitable for home use, at least) with an external faceplate with a little "Read only/Normal" switch on it. If you want to see what that weird attachment does, just flip the switch, and it becomes *physically* impossible for the drive to write data to itself. I can see so many applications of this. Much more convenient than running a webserver off a CD drive too, because changes don't involve re-burning everything.

I've fallen in love with the idea Totally unwritable drives, unless the crackers manage to write software to emit radio waves that reprogram the hardware circuits of the drive.

(If the drive already exists, let me know!)