For rapid prototyping work, Adobe Illustrator has basically been the only game in town. Occasionally, a new UI design toy will start getting some buzz, but the process of trying to adapt into a new workflow is a pain, especially when most don't live up to their hype. Recently, after enough people insisted that they'd found a home for prototyping, I decided to push through another curve and try Sketch by Bohemian Coding. I haven't looked back.
Sketch is one of the most perfectly priced, perfectly designed tools I've ever used for UI work. Designers have been waiting years for Adobe to get their act together in the area of UI design (and a few other areas as well), but instead we got expensive subscription pricing. Now my worst fear, and that of many other voices on the web, is that Sketch will be acquired by them. Stay strong Bohemian!
Another great tool I've started to really warm up to is Semantic-UI. Semantic is a lightweight front end framework powered by LESS and jQuery that leverages a natural language syntax. However, the community around Semantic is still a bit light, compared to frameworks like Bootstrap. One thing I've been struggling to find is a good resource of Semantic-UI elements for quick prototyping in Sketch, so I decided to build my own and share it.
Here you'll find a fairly comprehensive UI Kit for Semantic-UI. Not every element is covered, but it's a good start and I'll continue to update and support it for new versions in my available spare time. I also ported it to Illustrator for those who may not use Sketch, however, since it's not Adobe native there are obvious variances. Enjoy!