What is a good, simple and free HTML editor for Windows?

Notepad++ is good. Visual Web Developer Express is free as well, if not a bit more heavy weight for just HTML.

Try html-kit.
Or Komodo Edit.

Nvu and/or KompoZer. I thought Nvu was dead, but perhaps not. KompoZer supposedly picked up where Nvu left off. I've used them both and I'd say they certainly qualify as "simple and free".