sed or awk? They're both mainly used alongside some form of shell (BASH/CSH/ZSH) on the command line. A dieing portion of people write whole scripts in them.
The only thing I use sed for these days is quick replacing of characters on the command line or simple regexps:
echo 'Hello there my friend!' | sed 's/my/old/g'
The output of that is "Hello there old friend!". You get the idea.
AWK I have around 5 seconds worth of experience with so i'm not going to discuss
