Before Flying Wild Hog remade Shadow Warrior in 2013, the '90s classic hadn't been thought about for a while. It's considered up there with the true greats like Doom and Wolfenstein, and now you can download it for free on GOG and Steam.
It's DRM-free, so as soon as you download it from the website, it's yours forever. It will also run through the GOG Galaxy client if you so choose. GOG have made the game playable on modern systems too, so no need to go digging out your copies of Windows 98.