Some software that Twitter has put out:

[1] Heron, a realtime, distributed, fault-tolerant stream processing engine - https://github.com/twitter/heron

[2] Finagle, a fault tolerant, protocol-agnostic RPC system - https://github.com/twitter/finagle/

[3] FlockDB, a distributed, fault-tolerant graph database - https://github.com/twitter/flockdb

[4] Gizzard, a flexible sharding framework for creating eventually-consistent distributed datastores - https://github.com/twitter/gizzard

[5] Twemcache, a Twitter fork of Memcached - https://github.com/twitter/twemcache

[6] Twemproxy, a fast, light-weight proxy for memcached and redis - https://github.com/twitter/twemproxy