http://www.ecb.europa.eu/pub/pdf/other/virtualcurrencyschemesen.pdf"Different algorithms, i.e. the mathematical procedure for calculating and processing data
(it determines the speed at which the next block set of transactions is generated, how coins
are released, etc.).13 Despite being a very changeable attribute, two major algorithms can currently
be identified: SHA-256 (e.g. Bitcoin, Peercoin, Namecoin, Mastercoin) and Scrypt (e.g. Litecoin,
Dogecoin, Auroracoin), which could be described as an extension of the SHA-256 algorithm but
requiring more physical memory. In contrast to the former, for which specialised equipment for
mining was developed, deployed and therefore also needed to still be successful as miner, the
latter allowed miners to perform their activities with regular hardware. Currently, efforts are put
into using the X11 algorithm for reasons of higher cryptographic security and lower processing
costs, so it is quite possible that the algorithms mentioned above will be replaced shortly. "
Apparently X11 is going to replace SHA-256 and Scrypt
