KanColleTool (aka KCT) is a data viewer for KanColle, and a part of KanColleTool.
It takes data collected by the Viewer or MacViewer, and extracts information out of it.
Some of it is just nice to have when the game isn't running (or when it's just not onscreen), while some of it isn't visible in-game at all. It also provides notifications for things like constructions, reparations and expeditions.