Some GIMP Tips

Eliminate a solid background behind text or a simple figure:

For instance, if you have a solid white background with antialiased text on it, and you want to keep the text but change the background to transparent:
The Color to Alpha plug-in (in the Colors menu) will usually do a nice job, much better than Select by Color. If you need to touch up small areas, try drawing with the Paintbrush tool, with the foreground color set to the background color you want to erase and the tool's Mode set to Color Erase. (Make sure you've enabled alpha on the layer.)

Center a Layer:

Cut, then paste. The pasted layer comes out centered. (Unfortunately this loses text information, so if it's a text layer this isn't an ideal solution.)

Copy a layer, then paste it right on top of the previous version:

Do Layer to Imagesize before copying. Then copies of the layer will overlap the original.

Quick color picker:

When in a drawing tool (e.g. Paintbrush), pressing Ctrl temporarily switches to the color picker (eyedropper) tool to change the foreground color.

Make a pixmap brush that uses the current foreground color:

Before saving as .gbr, make sure your image is greyscale with no alpha channel. If you have any transparency, replace it with white, then "Flatten image" before saving.

Preview your image on top of a browser or other app:

Filters->Animation->Playback (yes, even for a single non-animated image), then click Detach. You'll get a detached window showing the image without any window borders or menus in the way.