ShadowNet is a program made by Seymour Birkhoff. ShadowNet is frequently used by Birkhoff throughout the series in order to access different databases but is only directly mentioned in Season 1 Episode 2: 2.0 and in the Season 4 Finale:
In 2.0, it is revealed by Birkhoff that ShadowNet has access to every government and law enforcement agency in the world. Michael remarks that there could be a better name for it, but Birkhoff states that since he is the creator, he can call it whatever he likes.
In "Canceled" it was revealed that Sonya convinced Birkhoff that the right thing to do is to open-source ShadowNet to the world.