Any free software for writing HTML?
HTML is a simple markup language and doesn't, arguably, require an IDE like Xcode. I'd recommend anyone starting to learn HTML (and related web tools like CSS and JavaScript) to start with a simple text editor. Learn your skills manually before relying on an app to speed up the process. Most of the time you'll find you can do a lot faster with a really good text editor than an all-in-one IDE. For something free on Mac try TextWrangler.

To be honest there aren't that many amazing and free editors/IDEs for websites on Mac OS. For some money there are a lot of amazing tools such as Sublime Text, Coda and Espresso to name a few. If you just need it for a short time it's not likely worth it however all of them currently provide some form of trial period.

Edit: Changed scripting language to markup language. Not sure what I was thinking.