The following are tools that can be used to model UI / System Mock-ups:

http://wamer.net/sketch/
https://mockingbot.com
https://www.mockplus.com
https://balsamiq.com
http://pencil.evolus.vn/Default.html
http://framebox.org
https://www.fluidui.com
https://g00glen00b.be/prototyping-spring-boot-angularjs/ (using Spring Boot & AngularJS for prototyping - advanced, requires Java coding)


See also:
https://blog.prototypr.io/the-7-best-prototyping-tools-for-ui-and-ux-designers-in-2016-701263ae65e8#.vddd5w87x
http://www.creativebloq.com/wireframes/top-wireframing-tools-11121302