The No-Signup Tools categories of interest to readers of this blog will probably be teaching, writing, and productivity. It was in those categories that I found helpful tools like Math Homework Generator, Egg Timer, and saw an old favorite called Hemingway App.
Here's a demo of Hemingway App.
Applications for Education
Web tools that don't require registration or other personal information can be great for students who don't have email addresses and they can be great for those of us who just don't want to give our email addresses to yet another website.
No-Signup Tools is good, but it isn't specific to education. For a similar resource that was specifically created with teachers and students in mind, take a look at Nathan Hall's list of No Registration Needed Tools.