Although the Humble Bundle 9 is a ridiculously good value, its games are high-profile enough that some of us have once again run into the issue of "I own most of these titles already. Multiple times over, in some cases." A good problem to have, I'll admit. There's another Humble sale running this week, one that compiles a handful of "retro shooters."
Pay-what-you-want pricing gets you the old-school shooters Serious Sam HD: The First Encounter, Serious Sam HD: The Second Encounter, Shadow Warrior Classic Redux, and Duke Nukem 3D: Megaton Edition. Slick. But you should consider putting forth six dollars or more to also receive Hard Reset: Extended Edition and the absolute classic System Shock 2.
Even without mods, System Shock 2 holds up fairly well. And, thankfully, there's a whole mess of them to further improve the experience for new players.