It partly depends on how you define 'still works'. The 100" Hooker scope is still doing research but only after a major upgrade.
http://en.wikipedia.org/wiki/Mount_Wilson_Observatory
The great Birr Reflector (aka The
Leviathan of Parsonstown) is operational but only after a complete rebuild, including new mirror, tube and drive. (BTW a few years ago it wasn't operating due to a problem with the drive, but I imagine that is fixed now.)
http://www.birrcastle.com/#
It is also interesting to spot deliberate?? mistake at
http://labbey.com/Telescopes/Parsontown.html
A useful list of old scopes is at
http://en.wikipedia.org/wiki/List_of...s_historically