Rainbow Six: Rogue Spear (PC) (CD-ROM)
In my opinion this is still by far the best tactical shooter available, far better than even its own sequels. The unfortunate reason for this is how it's sequels upgraded (or in my opinion downgraded) to the Unreal engine, which is incapable of providing the tactical environment that this game acheived (although some of the advancements were really great). So this is an old game, but its still a great one. Network play is also very enjoyable, we used to play this one a lot.
Now before you say why sci-fi? Since the clear motive here is tactical shooting, its acheived through a bit of creative narrative about terrorist activities and was at the time set in the future.
Onto the link, this has many sources but its personally unverified. I have this on retail so never downloaded a version myself.
ed2k: Tom.Clancys.Rainbow.Six.Rogue.Spear.iso [516.76 Mb] [Stats]