BeyondCompare (cited in the original thread), especially the folder comparison feature. Saved my sanity when our team's shared directory stopped syncing silently on my laptop and I had to figure out which files I needed to sync manually.
WizTree (https://antibody-software.com/web/software/software/wiztree-...) is a freeware Windows utility (man, typing this took me back to 1997) that lets you quickly see which files are hogging your disk space. Think "df GUI for Windows". Especially useful to track down large application files hidden in the depths of system folders.
Meld [0] (written with GTK so available on Windows, Linux, and possibly Mac but I don't know about that last one) is another tool in the first category.
WinDirStat [1] (Windows) for the second. It is also available in the super-useful PortableApps format [2], which is always nice.
WizTree (https://antibody-software.com/web/software/software/wiztree-...) is a freeware Windows utility (man, typing this took me back to 1997) that lets you quickly see which files are hogging your disk space. Think "df GUI for Windows". Especially useful to track down large application files hidden in the depths of system folders.