new items show up at the bottom
Nearly all image operations are performed by right-clicking on the image.
You can adjust the selection range for fuzzy select by clicking and dragging left and right.
You can change the name of a layer by double-clicking on the layer in the Layers dialog box.
The layer named "Background" is special. You can't add transparency or a layer mask to it. However, if you rename it, it will behave like a normal layer.
You can perform many layer operations by right-clicking on the text label of a layer in the Layers dialog box.
The file selection dialog box has command-line completion with Tab, just like the shell. Type part of a filename, hit tab, and voila! It's completed.
When trying to save files as GIF, XPM, or any other indexed color format, you need to convert the image to indexed using the Image menu.
Shift-click on the eye icon in the Layers dialog to hide all layers but that one. Shift-click again to show all layers.
Ctrl-click on the layer mask's preview in the Layers dialog toggles the effect of the layer mask.
Alt-shift-click on the layer mask's preview in the Layers dialog toggles viewing the mask directly.
Shift-click with the Bucket Fill tool to have it use the background color instead of the foreground color.
Click and drag on a ruler to place a Guide on an image. All dragged selections will snap to the guides. You can remove guides by dragging them off the image with the Move tool.
You can reassign shortcut keys on any menu by bringing up the menu, selecting a menu item, and pressing the new shortcut key combination. This is dynamic and is saved when you exit GIMP.
Control-drag with the Transform tool in rotation mode will constrain the rotation to 15 degree angles.
You can adjust and re-place a selection by using Alt-drag.
Selections can be constrained using the Control and Shift keys.
All the old channel operations have been replaced with the more powerful and flexible Layer and Layer Mode operations. They may take getting used to, but they are simply a better way to operate.
If your fonts turn out blocky, that's because they're not scalable fonts. Most X servers support scalable Type 1 Postscript fonts. Download them and install them.
You can use Alt-Tab to cycle through all layers in an image (if your window manager doesn't trap those keys...)
When using a drawing tool (Paintbrush, Airbrush, or Pencil), Shift-click will draw a straight line from your last drawing point to your current cursor position.
You can use the middle mouse button to pan around the image, if it's larger than its display window.
Brushes and patterns are viewed in small sections. You can drag them around inside their little cells to view the whole brush or pattern.
When installing a new version, be sure to delete your ~/.gimp directory first.
Mo Oishi sends this tip (he sent a few previous ones too): To create a perfect circle, hold shift while doing an ellipse select. To place a circle precisely, drag horizontal and vertical guides tangent to the circle you want to select, place your cursor at the interesection of the guides, and the resulting selection will just touch the guides.
You can get the Tool Options menu to open by double-clicking any button in the toolbar.