SASS + Susy + Bourbon for me

I just use SASS with Bourbon and Neat. Allows me to modularize my stylesheets and build components piece by piece when necessary.

In the previous couple of weeks, I was in the situtation like you. I'm developing and designing in the browser like 75% of my work time, therefore, I didn't want to leave the CSS for a long time because I was afraid of losing my real-time design workflow. I open up DevTools, change the rule,see the result and the code is there already. Just press Cmd+S/Ctrl+S and you're done, you've finished your job there.

Thereafter, I decided to use a preprocessor because I feel like I'm loosing the maintainability of my code. If they want some visual changes I need to add some couple of hundred lines and realized I've done that already. It's a totally waste of time. That's how I started css compiler.

I've started to use LESS and suggest you to do it. I suggest because you can see many solid examples like Bootstrap, Semantic UI, Kube, Schema so you can learn how to define properties professionally. Moreover, you can borrow their grid system, helpers, forms etc and build your product on that. For example, currently, I selected Kube because it is one of the clean UI I've just discovered and the output code length is relatively small.

PS: Many browsers support CSS sourcemaps which gives you freedom to write code in the browser and see the result without refreshing the page. This is the actual reason I switched to CSS compliers.

SCSS + Compass + self made responsive grid system

Sass & Compass usually, but also been starting to use Autoprefixer with Gulp.

Almost all homespun CSS.

I use Sass(SCSS). I use Jeet as a grid. I have a few mixins that I carry from project to project. If I have a pain point I look for a tool or build something and just put that inside my toolchain. I let gulp build everything for me... I'm pretty happy with my tooling but it's always evolving.