Marco.org

In Tumblr’s Preferences screen, you can specify Markdown as your editor instead of the automatic HTML-generating WYSIWYG view:

Edit posts using: rich text editor / plain text/HTML / Markdown

Markdown is a plain-text editing language optimized for quick writing and easy reading. You can mix in HTML if you want. Inline styles such as italics and bold are easy, plus some other conveniences:

• Lists and block-quotes are dead-simple.
• Double-dashes and smart-quotes are automatically converted — just type their “plain” versions.
• It’s a plain text field instead of the crazy iframe hacks needed for WYSIWYG editors, so all of your browser’s native helpers apply, including keyboard shortcuts and Firefox and Safari’s spell-checkers.

The only downside is that it fills up with HTML on reblogs, so you have to be a bit of an HTML geek if you reblog a lot. But give it a try: you might like it.