Project Endgame was a viral computer program originally created by Geordi La Forge that was originally designed to be implemented on the captured Borg drone later nicknamed "Hugh" as an attempt to wipe out the Borg Collective. Ultimately, this plan was abandoned by USS Enterprise-D Captain Jean-Luc Picard as genocidal, making the United Federation of Planets no better than the Borg.
The virus itself worked by showing the Borg an impossible geometric shape which the Collective would try to recreate. This would lead to their destruction as the Collective would devote more and more time and computer memory to solving an insolvable problem, which would ultimately cause the Collective to lose cohesion and disband. (TNG episode: "I, Borg")
In 2380, a colossal Borg Cube attacked Earth and Project Endgame was implemented as a fallback plan by Captain Picard and Seven of Nine, working against the Borg threat on the Enterprise-E. With the destruction of the planet killer and the loss of the original plan, Seven of Nine attempted to upload the Endgame virus into the Collective by allowing herself to be assimilated. However, the Borg Queen, formerly Kathryn Janeway, utilized a firewall to prevent the virus from spreading throughout the Collective but was stopped when the long dormant Janeway personality re-emerged and allowed Endgame to infect the Cube, destroying it. (TNG novel: Before Dishonor)