I've been having a look through other questions on the site and can't find one listing out some of the most widely used (or subjectively best) command line tools.
Having just installed Mercurial I'm interested in what command line tools others are using.
So what command line tools do you use and which ones do you prefer? Some reasons for why you use them and what makes it such a great tool would be really useful.
I'm currently on Windows so suggestions for Windows based command line tools would be a great help.
UPDATE:
What I'm really looking for is some software to replace the built in cmd.exe in Windows. There are lots of ways we can use this software however I'm really just interested in what replacement tools people are using. I find cmd.exe quite hard to work in and am looking for a good alternative.
UPDATE 2:
So it turns out that my wording has been slightly off. The thing I'm really asking about is a Shell or Command Interpreter. Thanks to everyone who's answered. Very helpful responses.