A Java-based powerful disk usage utility. It outputs a table and a tree (both can be sorted). TinyScan is able to export the result to Excel, Access, MySQL and it can also scan remote computers. It is free and open source.
SynchroMike is a two-way synchronisation program which allows the user to synchronize two directories. It comes with a handy user interface which displays differences between both storage locations as a tree.
Update: This page is deprecated, find the new program at <a href="http://syncarus.net">syncarus.net</a>