double space a file
sed G file

double space a file which already has some blank lines in it
sed '/^$/d;G'

triple space a file
sed 'G;G' file

undo double-spacing (assumes all even-numbered lines are always blank)
sed 'n;d' file

insert a blank line above every line which matches "regex"
sed '/regex/{x;p;x}'

insert a blank line below every line which matches "regex"
sed '/regex/G'

insert a blank line above and below every line which matches "regex"
sed '/regex/{x;p;x;G}'

number each line of a file (like grep -n "^")
sed = file | sed 'N;s/\n/:/'

number each line of a file (like cat -n)
sed = file | sed 'N;s/^/ /;s/\n/\t/'

number each line of file, but only print numbers if line is not blank
sed '/./=' file | sed '/./N;s/\n/ /'

count lines (like "wc -l")
sed -n '$='

delete leading whitespace (spaces, tabs) from front of each line
sed 's/^[ \t]*//'

delete trailing whitespace (spaces, tabs) from end of each line
sed 's/[ \t]*$//'

delete BOTH leading & trailing whitespace from each line
sed 's/^[ \t]//;s/[ \t]$//'