DoI shares many of its predecessor's traits, such as a tactical bent and eight player online co-op, but it's set in World War 2 instead of the modern era.  Originally created in 2015 as the aforementioned mod, it eventually made its way to Steam's Early Access program.  Today, it moves out of Early Access and into a full release.  As with Insurgency, the developers will probably continue adding content and making minor tweaks as time goes on, but the game is now considered to be in a largely complete state.