Post-WW2, the allies sold enigma machines to developing countries, without mentioning they had broken the code.

Source: https://en.wikipedia.org/wiki/Enigma_machine

Note: some people speculate the purchasing countries knew this and accepted the risk (https://www.reddit.com/r/AskHistorians/comments/5cpqrc/after_ww2_the_uk_government_sold_enigma_machines/).

The British kept radar secret during WW2, and attributed pilots' ability to shoot down planes in the dark to vitamin A.

Source: https://www.smithsonianmag.com/arts-culture/a-wwii-propaganda-campaign-popularized-the-myth-that-carrots-help-you-see-in-the-dark-28812484/

• Nuclear weapons
• Dangerous biotechnology
• Cultures of dangerous pathogens (seems a bit weird to call this a secret, but it has a lot of attributes of secrets)
• Tools and knowledge to build better gene-editing technology
• Concrete applications of existing technology that could be highly dangerous
• Chemical weapons
• Drone manufacturing technology?
• Methods of terrorism
• Potential critical weakpoints in infrastructure
• Other terrorism ideas