Proof of concept (POC) is a realization of a certain method or idea in order to demonstrate its feasibility, or a demonstration in principle with the aim of verifying that some concept or theory has practical potential. A proof of concept is usually small and may or may not be complete.
In software development, the term proof of concept often characterises several distinct processes with different objectives and participant roles: vendor business roles may utilise a proof of concept to establish whether a system satisfies some aspect of the purpose it was designed for. Once a vendor is satisfied, a prototype is developed which is then used to seek funding or to demonstrate to prospective customers.Wikipedia
We love to learn new technologies, trends and tools. That's why we have such sites - PoC's.
Until we will have few great projects in our portfolio on selected technology or framework, we can play with such "dummy" sites.
Every site from list below can prove, that we know this technology, can create and customize project on it, and know how it works from inside.