General
Antiquers Forum: The forum members have a lifetime of antiques expertise covering a wide range of subjects. There is a strong US presence, but Antiquers also has members from all over the world giving global expertise. We have found them invaluable for insight and identification of items with the added bonus of a strong sense of support and community. Well worth registration for full functionality
https://www.antiquers.com
Glass
20th Century Glass Encyclopedia Comprehensive guide to modern glass https://www.20thcenturyglass.com/glass_encyclopedia_home.htm
Great Glass Useful guide to identifying some makers marks
http://www.great-glass.co.uk/glass%20notes/marka-c.htm
The Paperweight People:
http://www.pwts.co.uk/index.shtml
Image Search
Conducting a reverse image search can be a good way of identifying an obscure object or painting/print. Each work by uploading an image from your computer and the search engine then tries to find relevant matches.
Tineye: Our preferred engine as it seems to identify only relevant matches
https://www.tineye.com
Google: We have found this less useful as it appears to find several non-relevant matches
https://www.google.com/imghp
Patent Searches:
Espacenet
https://worldwide.espacenet.com
US Patent Office
https://www.uspto.gov/patents-application-process/search-patents
Postcards
Playle Deltiology: US orientated
https://www.playle.com/realphoto/
Edinphoto: Scottish orientated
http://www.edinphoto.org.uk