Great utility that makes it trivial to play the official Doom maps, especially Master Levels for Doom II, which was a hassle to get to work in a source port, and works as it should (starting out with only the pistol on each level).
It even incidentally let me know about a Doom campaign that I had no idea that I owned, No Rest for the Living, which comes with the Doom 3 BFG edition on PC.