No One Lives Forever, launched in 2000 became one of the most popular and loved games of its time, mainly for its spoofing of the spy genre in an Austin Powers style.
The hilarious dialogue alone is worth playing the game for, not to mention first person stealth shooter that it really is. But the game has a weird history in that neither Warner Bros or Activision can agree on its ownership as outlined in this article by Rock, Paper, Shotgun.
Consequently, the game has been patched up by the community to run on modern systems and can be downloaded from NolfRevival.
As to the legality of this download, it's very much a grey area and it could be argued that, should WB, 20th Century Fox or Activision protest against the game being made available for free, the implication would be that they had sorted out the legal ownership issue which, up to now they haven't.