Currently in beta, at https://chat.openai.com/chat
You create an acct (you need to verify your phone #) and you can chat with ChatGPT.
https://www.youtube.com/watch?v=V2RoqUr0qDU&t=74s&ab_channel=TwoMinutePapers <-- Great video summary of what ChatGPT does.
ChatGPT is in 'public preview/prerelease'. That means there's no official API access yet. OpenAI are testing it out before releasing.
For the impatient, we can create programmatic access by simulating a session in a browser where we ask questions and get responses, and exposing our own API.
There's a Discord community here: [ChatGPT Discord Guild]
There's at least half a dozen working (or partially working) solutions already available. See below table