Seamless textures!

The Texturize plug-in is an amazing tool for creating seamless tileable textures. It can also generate larger images from only a small sample.

High pass filtering

The high-pass filter plug-in is designed to smooth out the average brightness of an image, which is sometimes required before attempting to make a seamless tileable image.

Remove a selection from an image

The Resynthesizer plug-in is another seamless tile creation tool. Although it isn't nearly as impressive as the Texturize plug-in for creating seamless textures, it does a wonderful job of removing regions from an image (available from the scriptfu/enahnce/smartremoveselection menu). See this Newsforge article for more information.

The TextureOps plug-in is another texture tool created by the same author, which is supposedly faster, although I found it produced less desirable results.

Brushed metal and more

The brushed metal plug-in creates very realistic looking metal textures. Also on the same page are plug-ins for creating sand, canvas and even cow textures! Here are some tutorials for using these plug-ins.

Yet more great metal textures

A little older, yet the Brushed Metal plug-in is another great metal texture creation tool.

More plug-ins!

See the GIMP Plug-In Registry for a complete list of plug-ins for the GIMP. You can also find another list of plug-ins at the Devian Art 'gimp actions' page.