Primecoin

Primecoin is a cryptocurrency that implements a proof-of-work system that searches for chains of prime numbers.
Primecoin was launched in July 7, 2013 by Sunny King, who also founded Peercoin.
Unlike other cryptocurrencies, which are mined using algorithms that solved mathematical problems with no extrinsic value, mining Primecoin involves producing chains of prime numbers. These are useful to scientists and mathematicians and meet the requirements for a proof of work system of being hard to compute but easy to verify and having an adjustable difficulty.
Shortly after its launch, some trade journals reported that the rush of over 18,000 new users seeking to mine Primecoin overwhelmed providers of dedicated servers. It was ranked as being one of the top ten currencies before 2014.
Primecoin has a block time of one minute, changes difficulty every block, and has a block reward that is a function of the difficulty.