Sed

Change double-spaces in a text file to single spaces
cat redpine.txt | sed 's/ \+/ /g' > redpine2.txt

Display all subdirectories in a tree format
ls -R | grep ":$" | sed -e 's/:$//' -e 's/[^-][^\/]*\///g' -e 's/^//' -e 's/-/|/'

Make a file lowercase, strip out punctuation, and list all the resulting unique words
cat cotus.txt | sed -e $'s/\t//g' -e "s/punct:\+//g" -e "s/./\L&/g" | tr ' ' \\n | sort | uniq

Prepend an asterisk to every line in a file
sed -ne 's/.*/* &/p' enterprise.txt > enterprise2.txt

Remove tags from HTML file
sed -e :a -e 's/<[^<]*>/ /g;/</{N;s/\n/ /;ba;}' CleanPosts.html