Ethereum

In order for Bitcoin to operate, it runs programs to perform the calculations, so it effectively is a blockchain that can do specific programming tasks. Ethereum ran with this idea and built a blockchain technology that could act as a general purpose computer. In short birthing the embodyment of programmable money.

Want to trigger a payment on a schedule and have both parties confident it gets paid. Or more famously create new limited tokens that have immutable rules of issuance that can be used in many ways like shares in a company (the classic ICO). Although ICOs have been the main use of Ethereum, and many individuals are looking to see what uses a mix cryptocurrency and applications will enable.

That said with new technology there are lots of challenges. Interesting challenges ahead for Ethereum include handling many orders of transactions greater than are seen today, as well as ensure that the network is resistant to the many modes of attack that it has faced.