According to TechCrunch.com, some of the more prominent programs you might recognize being used are familiar enough. Those such as Ruby, Scala, Java, C/C++, and other various tools. And some you may have heard of, such as Cassandra, the P2P structured storage network Facebook began in 2008. While some of them you may have not, such as Murder, the code deployment project that uses Bittorrent.
Also, if you’re interested in taking your research a step further into the world of Twitter open source projects, you can always visit the official Twitter Engineering blog to gain more insight.