Markdown
When I was starting this site and designing the software that powers it I decided that I didn’t want to use raw HTML for writing posts. After 10 years of authoring HTML I still find it visually obtrusive, and somewhat distracting when trying to focus on writing. I considered both Textile and RST but ultimately settled on Markdown. The implementation I use is Michel Fortin’s PHP Markdown, which has worked flawlessly.
I find that, as advertised, Markdown is great for writing text (and hypertext). The source is very readable – which makes the writing process a lot easier, especially for longer posts – and I almost never need to write straight HTML.
I use Markdown for the comment engine, too, though some of my HTML-tag-stripping is problematic there. I suppose I could improve the user experience with live comment previewing…