Ubuntu : This does not look like a tar archive

The file you downloaded was a .tar.gz. (A compressed .tar file)

You can use

tar -zxvf casperjs.tar

But actually you should have downloaded it as a .tgz like this:

sudo wget -O casperjs.tar github.com/n1k0/casperjs/tarball/1.0.0 –O casperjc.tgz
sudo tar -zxvf casperjs.tgz

The -z stands for:

-z, --gzip, --ungzip filter the archive through gzip

to see what you got:

file casperjs.tar

and then to see its content, if it IS a tar file:

tar tvf casperjs.tar
tar tvzf casperjs.tar  #if that was a gzip-ed tar file
tar tvbf casperjs.tar  #if that was a bzip-ed tar file
tar tvZf casperjs.tar  #if that was a compress-ed tar file  #now very unlikely...
 #note that after the 'f', you need to have a SEPARATOR (space, or tab) followed by the FILENAME. 
 #ie, you can't place an option after the f
 #ex: "tar tvfz something.tar.gz", would try to open file "z" instead and find "something.tar.gz" inside it...

then change the 't' into 'x' to extract (I recommend to first use 't', though...)