What is the most used tool in my daily work life? The Java compiler? IntelliJ IDEA? Perl?
Actually it is Mark Edgar's PuTTYcyg, a PuTTY patch for Cygwin terminal. This runs BASH from Cygwin inside a PuTTY xterm-ish window, making my daily work on Windows rather more productive.
You see, I'm a command-line sort of guy. And BASH is indispensable to my habits; hence Cygwin. And the default DOS box terminal is lame.
Using PuTTYcyg is a cinch. Download, unpack and add a shortcut to your desktop for C:\puttycyg-20070320\putty.exe - (or wherever you unpack to; the important part is the trailing solo hyphen). I usually change the icon to the Cygwin one (%SystemDrive%\cygwin\cygwin.ico, YMMV).