I write HTML + CSS (no preprocessor) + JS, and when the situation needs something like:

• scoped styles
• element queries
• selectors like $parent, or $prev
• old browser support (IE8, IE9, etc)

I'll also add EQCSS to the list :D With HTML + EQCSS + JS I can not only skip most of the pre & post-processing tools, but I find I can cut down, or eliminate most of my frontend plugins as well!

I use Compass SCSS and LESS on my daily work but actually prefer to use Stylus, is way faster with autoprefixer (no need to add @include or mixins manually) and the functions are more intuitive and reasonable but also really fast to setup and cleaner to the eye.

I occasionally use stylus.

I actually combine Sass with PostCSS (mainly AutoPrefixer).