Java File Explorer
This is a tree for exploring file structures written in Java. The idea is similar to what you see on the left hand side of Windows File Explorer. The real interesting feature here is the use of native widgets for each icon. The only problem at the moment is speed when the tree is expanded. Custom widgets have to be looked up which can show a noticeable delay if the expanding node contains lots of children. This is obviously why Swing moved away from native widgets to begin with but with some work this should be able to be sped up to satisfactory speeds.
(Ugly) Source code can be found here.