A quick update for those of you who are using GIMP 2.8 under Ubuntu 12.04 or 11.10: Otto's PPA which you've probably used to install GIMP 2.8 has been updated recently and it now includes GIMP Plugin Registry, recompiled to work with GIMP 2.8 (see how to install GIMP 2.8 in Ubuntu, HERE).
GIMP Plugin Registry is a collection of hundreds of plugins for GIMP which includes GIMP FX Foundry (a huge collection of scripts for GIMP), Liquid-Rescale, Safe For Web, Separate+, Elsamuko (which includes effects such as "Obama Hope", "Movie 300", antique photo border and more), Warming and Cooling Filters and others.
To install GIMP Plugin Registry for GIMP 2.8 in Ubuntu 12.04 or 11.10, use the commands below:
sudo add-apt-repository ppa:otto-kesselgulasch/gimp sudo apt-get update sudo apt-get install gimp-plugin-registry
For GIMP 2.6.x, you can use the WebUpd8 PPA to install GIMP PLugin Registry (it's also available in the Ubuntu 12.04 Precise repositories).