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Notepad++

But if you don't want to code by hand and you want a good WYSIWYG editor, I've heard good reviews of NVU. Personally I code all of my HTML, PHP and Javascript by hand so I use Notepad++ for almost everything. Dreamweaver comes in handy for validating my HTML and CSS without having to go to the w3c validators, but it's a pretty expensive tool (I only use it because I have it).

Nvu is open source. You can use that.