To make it on this list, a site must be:

Free. There may be paid versions, but there is an option to use without paying.
Without the need for students to give their email address. Teachers may have to register, but there is an option for students to either sign-in or use without giving their own email address or log-in with their Facebook/Google/Twitter/etc. account.
For the most part, interactive or creative. The ideal tool allows the student to express themselves through creation, but some of these tools are for teachers only.
Completely online. There should be no need to install something to make it work. Some of these sites require things like Flash or Java which are on most desktop and laptop computers, but are normally not accessible on a tablet or phone.

Blog post by Nathan Hall list-of-webtools-no-registration-needed-for-students

Nathan Hall Bio https://nathanghall.wordpress.com/about-2/