Below you can find a list of websites containing information on display tubes I found useful.
tube-tester.com: Possibly the most comprehensive online resource for display tubes, offering datasheets for nearly every display tube ever manufactured on their archive.
jb-electronics.de: Large collection of Nixie tubes and Nixie-related projects of which many are documented with detailed pictures.
jogis-roehrenbude.de: Documents many types of tubes, including display tubes.
lampes-et-tubes.info: Contains information on many obscure tubes.
nixies.us: Contains information on Nixies and some other display tubes. Also documents various projects using display tubes.
industrialalchemy.org: Detailed information on many, mostly American, display tubes.
swissnixie.com: Pictures of many rare Nixie tubes.
radiomuseum.org/: Contains information on basically any tube ever manufactured. Unfortunately relatively restrictive on accessing their content.
r-type.org: Contains information on many tubes including pictures, documents and historical adverts.
nixieclock.org: Large collection of Nixie and VFD tubes including some very rare specimens. They also sell high quality boost supplies for driving Nixie tubes.
asahi’s website: Collection of Japanese Nixie tubes including some very rares ones.
solorb.com: Website dedicated to documenting every magic eye tube ever made.