GNU Bash. Most *nix developers use it every day and if they are like me, they must never cease to be amazed at the cool stuff you can do with it.
A simple thing today I tried out today, downloading and extracting a web-based tarfile in a one liner.
It's really simple.
$ tar zxv < <(wget -q -O - http://www.somewhere.com/atarfile.tar.gz) > atarfile/ > atarfile/file.txt > ...
So let's look at this statement in a little detail. Basically we evaluate the right side of the expression first. We're using wget to download, quietly (-q), a tgz file from somewhere on the internet and output the file's contents to stdout (-O -).
We use Bash's Process Substitution operator '<(' to create a Named Pipe, which effectively creates a temporary file descriptor, and then direct the contents of that descriptor into tar using the '<' file redirection operator.
Sounds complicated but looks simple.