Show HN is for something you've made that other people can play with. HN users can try it out, give you feedback, and ask questions in the thread.
On topic: things people can run on their computers or hold in their hands. For hardware, you can post a video or detailed article. For books, a sample chapter is ok.
Off topic: blog posts, sign-up pages, newsletters, lists, and other reading material. Those can't be tried out, so can't be Show HNs. Make a regular submission instead.
The project must be something you've worked on personally and which you're around to discuss. See these tips about how to present your work.
A Show HN needn't be complicated or look slick. The community is comfortable with work that's at an early stage.
Please make it easy for users to try your thing out, ideally without barriers such as signups or emails. You'll get more feedback that way.
If your work isn't ready for users to try out, please don't do a Show HN. Once it's ready, come back and do it then. Don't post landing pages or fundraisers.
To post, submit a story whose title begins with "Show HN".
New features and upgrades ("Foo 1.3.1 is out") generally aren't substantive enough to be Show HNs. A major overhaul is probably ok.
Please don't ask friends to upvote or comment. That's not ok on HN.
Be respectful. Anyone sharing work is making a contribution, however modest.
Ask questions out of curiosity. Don't cross-examine.
Instead of "you're doing it wrong", suggest alternatives. When someone is learning, help them learn more.
When something isn't good, you needn't pretend that it is, but don't be gratuitously negative.