My text editor of choice is Kate from the Kde project. It's simple and fast and feature sufficient. The default list of opened document files is a little bit rude. Particuraly if you have to open a files with the same name : it's difficult to differienciate one from each other.
A tree view is ideal in this case because each opened file could be visualy located in the tree and differenciated easily. When I work in a project for a while I visualy memorize the pattern of files and directory and find them quicker time after time.
So I had the need to create a treeview plugin for kate. I used the source code of the current Kde4 tree. Kate is located now in the kdesdk directory. Getting a working plugin was not hard but something usable is another thing.
I ran into a strange bugs when the QDirModel displayed by a QTreeView is filtered with setNameFilters : file selection may be lost, some directory may be closed without any reasons. This annoying bug remain in this plugin.
I have posted this plugin to the Kwrite mailing list but it's hard to get some feedback. Kate hackers seems to be very busy these days.