QDirStat

QDirStat scans a directory tree and reports where the space went. The upper pane is a sortable tree of subdirectories and files showing size, item counts and each entry's share of its parent, so a large branch can be opened level by level until the directory actually responsible appears. The lower pane draws the same scan as a treemap, where every file is a rectangle sized by its bytes, which makes a handful of oversized files stand out at a glance.

Beyond browsing, it groups the scan by file extension, so patterns such as accumulated logs, caches or video files become visible, and it can list the largest files found. Selected files and directories can be deleted or moved to the trash without leaving the window. The cleanup menu is user-configurable: each entry runs a shell command against the current selection, so routine chores specific to a machine can be added to it.

It is a Qt program and does not need a full KDE session. Scanning still costs one complete traversal of the tree, so the first run on a large disk takes time.