Hands down the perfect prototyping tool for me. It's simple yet powerful and also pretty intuitive. Most things that are mising shouldn't be part of a prototype anyway. This thing is genious. Atomic
Well...it used to be the industry-standard and I still use it for almost all of my professional work. People complaining about it's complexity know they can create custom layouts, right? Adobe Photoshop
I use it for a lot of professional work where I might need more than just a responsive grid. Has a great set of mixins and components and also some nice JS-modules. I love that its so style-agnostic. Really easy to add custom styling. Foundation
I use it for professional work where I know I need a lot ready-to-go stuff baked in like date-pickers and such. Not so easy-to go with custom styling though. Bootstrap