TexBrowser is an addon for browsing the game's textures while in-game. You can also modify the vertex color, and preview a clipped region. Finally, you can copy a texture's path for easy pasting into other parts of the UI by right clicking its entry in the browser tree.
I don't know if it's feasible, but it'd be kind of cool if clicking on a category (like tileset, or tileset->duskwood in your screenshot) would show tumbnails in the right side of the frame of all textures that are in the chosen category and all subcategories of that category. This would allow the user to do a visual search instead of a tedious text one.
I think I'm drawing on memories of map editors for Doom :)
Is it possible to download this somewhere? I'm using the addon SexyMap, which makes reference to TexBrowser, and I would very much like to see what textures I can use to make my own borders around the minimap :)