This could make for a very interesting topic, its not something I've really looked into. QT Tabbar and Findexer look interesting, shall certainly be trying QT out (why have tabbed Windows never occurred to me before as an idea worth looking up ).
Only things I have installed that would maybe be considered and explorer addon would be Eraser (5.7) which I use as a right click replacement for the standard delete, and Styler to pretty up the explorer toolbars. Tried Teracopy a while back but never found it particularly advantageous for my uses.