Tags that don't seem to fit in any other section ...
- Comment -
<!-- hello world --> - Anything between the comment tags are not displayed by browsers, so they are used documenting your html code, or just hiding sections of code you don't want to delete just yet.
- Blinking Text -
<blink> ... </blink> - Blinking text can be usefull when making things stand out, however overuse of this tag can actually cause a web page to appear ugly. I find blinking text very annoying.