Forgotten Technology

In this day of re-purposing and up-cycling, what forgotten technologies are there that could be re-imagined for today's use?  There's sure to be lists of forgotten inventions, innovations and discoveries that could be used today.  Someone somewhere built a trinket, a device, a contraption that could have value today in a new arena that was not even around when it was first created.  Where are these forgotten technologies?  How can they be found and what can they be used for?