sudo apt-get install par2
to install par2, also
sudo apt-get install unrar
for most files on usenet are archived in rar format.
is the way to get the repair going, it not only works with rar files but also with all other files if the par2files are added to the posted binaries.
For downloading binaries from usenet I recommend Pan.
To unrar a set of rar archives just do
unrar e first-rar-file.rar