Others have mentioned uses for these tubes. What they haven't mentioned is
that you will need a power supply delivering DC somewhere in the 150 to 250
volt range. It is absolutely vital that this supply have a transformer to
isolate you from the 115 V AC mains. Otherwise you'll likely kill someone or
yourself, since on side of the AC line is connected to ground and the other
is HOT with respect to every grounded object.

Junk boxes all across the world may be full of these transformers, but you
may have trouble finding one. In that case you could use a couple of 115 to
12-or-so volt transformers. One to go from 115 to 12 V AC, the other
connected "backwards" to go from 12 V AC back to 115 V AC. Now you have 115
V AC with neither side grounded - much safer.