zsh - especially its file matching abilities. For example:

 grep wibble **/*.(js|html) 

will search for "wibble" in .js and .html files in the entire directory tree from the current directory down. This is why I have not felt the need to switch to ack and almost never use find.

sed and awk.

ipython - not only for developing python code, but also for doing data processing tasks that are too complex to easily do in sed or awk.

I use head a lot when developing a complex pipeline as a quick check to see that it is doing what I expect without it running off and processing gigabytes of data.

seq to loop a fixed number of times in a shell script:

for x in `seq 10`; do
    echo $x
done

The combination of sort | uniq -c | sort -n is useful to get a count of how many of each unique string is in the input. For example to get the ten most frequent words in a file, along with their counts:

cat somefile.txt | tr -s '[:space:]' '\n' | sort | uniq -c | sort -nr | head -n 10