PROTEUS, the first expansion for Netrunner, also referred to as v2.1, was released 5 months after the initial release of Netrunner. It added a few interesting mechanics to diversify game play:
Runners and some Black Ops could now cause the Corp to accumulate so-called "Bad Publicity" counters. These counters worked like agenda points in that they could not be removed once you had one - but unlike agenda points, having 7 of them would cause the Corp to lose the game.
Runners were given a new Resource type that now gave Runners an edge that only the Corp had: "Hidden Resources" would enter the game face down and were not visible to the Corp until their ability was used; typically, this involved trashing the Resource.
Some things were also upgraded for the Corp. Many ICE cards featured abilities that allowed the ICE to become more powerful as things developed in the game - and with "World Domination", the Corp could now win the game with a single Agenda.
Proteus cards were sold in 15-card Boosters and are now rarities, sought after by many collectors due to its limited print volume.
Official Publisher Synopsis from the back of the box:
For years the data fort had cycled idle, a derelict from a corporate war. When Max Forward lifted its address, he expected a cakewalk: the fort was old, unmonitored. But then it's not every day that a Runner faces a revolutionary security experiment...
Discover the product of the new tech Forward brought to light: ice capable of transmuting into completely different countermeasures; smart icebreakers that can rewrite their own code. Proteus. Whether you're Runner or Corp, expect a few changes.
Blurb taken from: http://www.netrunneronline.com/set/proteus/