UI designing will be crazy with this app. Sketch is a mix of Adobe Illustrator and Photoshop in a lightweight (About 55MB) version. It is easy to use, has powerful vector editor, hundreds of plugins and has a wide range of resources. All UI design software supports sketch format. Note: Sketch it is only for Mac OS.
Download link: https://www.sketch.com/
Lunacy (For Windows) Free
Finally, found a Sketch alternative for Windows. It’s like a copy of Sketch app.Lunacy has native support for sketch files. Lunacy can view, edit and save sketch files. It is easy to use, has powerful vector editor and has all the necessary features. Lunacy updates regularly and it is getting better. I personally use this app since I use Windows.
Download link: https://icons8.com/lunacy
Adobe XD (Windows, Mac) Free
Adobe XD is full featured UI design tool, where you can create design, add animations and collaborate with others. XD is easy to use like Sketch, Lunacy. But It is heavy app and takes a lot of resources. XD has many unnecessary features. Also Adobe customer support is terrible.