Project I.G.I. (AKA: Sneaked, IGI is short for Institute for Geotactical Intelligence) is a series of tactical first-person shooter (FPS) PC game with realism elements and excellent graphics & sound effects design. In particular, the whole game comes with fantastic sandbox maps and playing methods, there are even dedicated servers built by fan players for online play. That's why it's so popular that it got numerous positive reviews from players around the world the time it came out (it has been 18 years as of this post, but AppNee is still willing to give it good praise).