OpenAI GPTs
GPT Tracker lists hundreds of curated GPTs. 
@NickADobos surveys some of the more popular GPTs > 3 weeks after OpenAi Dev day & GPTs announced > >Grimoire: 55,200+ chats Gif-PT: 7,500+ Shopkeeper: 6,300+ Total across 10 custom GPTs: 78,000+ > >Revenue: $2192
AgentOps Github top GPTs
ConvertAnything—convert images, audio, videos, PDFs, files, & more.
editGPT—edit any writing (like Grammarly inside ChatGPT).
Grimoire—a coding assistant that helps you build anything!
How to Build GPTs
Simon Willison explains (Exploring GPTs: ChatGPT in a trench coat? with multiple specific examples of how to use GPTs. via HN
For example, this “Dejargonizer” is defined like this:
Explain all acronyms and jargon terms in the entered text, as a markdown list. Use bold for the term, then provide an explanation. Mention the likely context for the term where appropriate. If a term could mean several things list each potential definition in a nested list.
List the least obvious terms first.
The first time you answer end with “Type ? for further explanation”—if the the user types “?” then provide explanations of any new jargon terms you used to explain the previous jargon.