One of the features of less is piping the file to be viewed through a filter program (usually called lesspipe.sh) before viewing it. This means that gzipped files can be decompressed, troff source formatted, HTML rendered into plain text, and so on. Typically lesspipe.sh is a shell script which runs different filters depending on the input filename.
The standard distribution of less does not include a lesspipe.sh, but there is one included in the less package from Red Hat (http://rpmfind.net//linux/RPM/rawhide/1.0/s390/SRPMS/less-378-2.src.html) (and related distributions) and another with Slackware. Wolfgang Friebel's lesspipe.sh (http://www-zeuthen.desy.de/~friebel/unix/lesspipe.html) is unusual in that it uses file(1) to look at the file's contents.
An open, cross-platform journaling program.
A scientific plotting package.