FARR plugins are written in a native language like C++. Thus to build plugins you need a compiler and know the desuate art of native code programming.
You can get the last version of the dll at : http://e.craft.free.fr/farr/FScript/
Take the version with the highest version number.
An help file that detail most of the functions is available at http://e.craft.free.fr/farr/FScript/help.html
There is many plugins written that you can find on the DC forums, in the plugin list, or on the CZB page : http://czb.dcmembers.com/
FScript original plugin was the delicious plugin, that allow to explore your bookmarks stored on the delicious website.
It is available at : http://e.craft.free.fr/farr/delicious/delicious.zip
Discuss it on dc forum : new FARR plugin Delicious
FSubScript is a plugin that allow to load multiple fsubscript plugins for FARR within a single fscript plugin. It allow for easier plugins creation, sharing and lesser resources use since the script engine is shared.
CZB made many improvements on the original FSubScript prototype. As far as I know the latest version is : http://czb.dcmembers.com/Plugins/FSubScript/FSubScript.zip
As for FScript the source is available at : http://github.com/ecaradec/fsubscript/tree/master
FFTab is a plugin that connect FARR to firefox and allow to list and switch tab from FARR. http://e.craft.free.fr/farr/FFTab/FFTab.zip
This plugin require that you install mozrepl first : http://repo.hyperstruct.net/mozrepl/0.2/mozrepl.xpi
Feel free to check http://wiki.github.com/bard/mozrepl for more information about what mozrepl is.
And FSubScript from the precedent link.
Discuss it on the dc forum : New FARR plugin : FFTab allow to switch firefox tab