Flat UI Pro is described as 'is made on the basis of Twitter Bootstrap in a stunning flat-style, and the kit also includes a PSD version for designers. Flat UI Pro contains a huge number of basic components, icons and glyphs'. There are more than 25 alternatives to Flat UI Pro, not only websites but also apps for a variety of platforms, including Self-Hosted solutions, Mac, Windows and Linux. The best alternative is Bootstrap, which is both free and Open Source. Other great sites and apps similar to Flat UI Pro are Materialize (Free, Open Source), UIkit (Free, Open Source), Semantic UI (Free, Open Source) and Purecss (Free, Open Source).