Plug-ins are extensions written mostly by GIMP users. Currently they're a bit of a mess and can often be hard to compile. I suggest you look at http://nuclecu.uman.mx/ federico/gimp/el-the-gimp.html for instructions on compiling and installing plug-ins. This page is quite comprehensive and contains links to all other known GIMP sites and tutorials. A nicely patched source tree of 0.61 is available here also.
You can also download a package of precompiled plug-ins for 0.54 from
http://worldserver.pipex.com/home/adam/gimp/plug.html. This
is the path of least resistance. Take it and don't look back!
There is an up again down again automated plug-in registry where plug-in authors register and upload plug-ins as they write them, and users can download them as they become available. Find this site (if it's up) at http://blank.pages.de/registry