GIMP Plugin Registry is a collection of many additional plug-ins for GIMP. It consists of plugins like GIMP FX-Foundry (one of the biggest collection of GIMP scripts), GIMP-Mask, Normalmap, etc. To install it on Ubuntu, perform this command:

sudo apt-get install gimp-plugin-registry
In Ubuntu 14.04 32 bit, this will download about 2 MB data. After installing, restart running GIMP to see changes. You will notice menu Filters will have new entries and new filters. You will notice also new FX-Foundry and Script-Fu menu. And some other stuffs (check menu Color, Plug-In Browser, and Procedure Browser).