According to Tom Preston-Werner, one of the GitHub founders, in his post “Open Source (Almost) Everything”, about the open-sourcing philosophy
Don’t open source anything that represents core business value. […] Notice that everything we keep closed has specific business value that could be compromised by giving it away to our competitors. Everything we open is a general purpose tool that can be used by all kinds of people and companies to build all kinds of things.
However, Hubot, previsouly a closed-source asset of GitHub, was eventually open-sourced in late 2011.
For the past year or so we’ve been telling people about Hubot […] So we decided to rewrite him from scratch, open source him, and share him with everyone.
Currently, the Windows Github client is not an open source software… but who knows, it might be open-sourced one day.