Memcached and Redis are in-memory key-value pair storing system, or in short, a “caching” system. Both of them open-sourced.
First a bit of history.
Originally developed by Brad Fitzpatrick at Danda interactive for their website LiveJournal, which was very popular in 2003. It was written in Perl at first, then written in C by Anatoly Vorobey (who was also employed at LiveJournal because of his work).
Now in 2015, this program has continuously proved its capabilities and is used by a lot of top websites such as: