GIMP 2.5.3 is a fresh snapshot from the 2.5 development series. It gives developers and interested users a sneak preview of the upcoming GIMP 2.6 release.
GIMP has several UI changes in this version, many of them offering relief to long standing issues.
A major change is the appearance of an "empty image window". This window is a placeholder where your image will eventually open, also a drop zone into which you can drag-and-drop your file and start working. You will also notice that the Toolbox menu is gone: it has been merged into the image window menu. Toolbox and docks are treated as utility windows, so if your window manager supports it, then your problems with docks and toolbox getting lost under other windows are over. Unfortunately, at this moment utility window hints only work correctly in metacity, the Gnome default window manager.
There are also a couple of nice usability changes concerning docks.